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In the manufacture of polyester fiber, first takes place the synthesis of the polymer, and subsequently the spinning and stretching of the fibre. The conditions to which these processes occur, such as temperature, drawing speed or time, have great influence on the fine structure of the resulting fiber (crystallinity and orientation). If variations of these conditions occur, voluntary or not, it may produce differences in the properties of the fiber. This is because the fine structure of the resulting fiber can present irregularities, ie differences in crystallinity and / or orientation. It is important to detect these irregularities in the structure as soon as possible, because the properties affected may be different: since an irregular deformation of the material until an irregular dyeing. To achieve the final textile product, it is necessary to carry out different processes. In each of these processes (warping, weaving, bleaching, dyeing, finishing, ...), energy, water and chemicals consumption, occur, whith the corresponding cost. The earlier the detection of irregularities involuntary, the lower the economic losses of the company. Differential solubility is a physicochemical technique that can reflect differences in the fine structure of the fiber. It informs about the compactness of the fiber. This is a relatively simple test that does not require high cost equipment, or complicated operation. Then, it is possible to use this technique in any laboratory quality control of a textile company. It can also be used when the fiber is still present as staple fibre. In the differential solubility test, the fiber contacts with a solvent/non-solvent mixture, under certain conditions of temperature and time, and the percentage of dissolved fiber is determined. The more crystalline and/or oriented fiber has the lower differential solubility. Traditionally in this test a mixture of phenol/tetrachloroethane (Ph/TCE), from 2.5/97.5 in weight for heat-treated fibers, and up to 30/70 samples at high heatseting temperatures has been used. The European Union, with the intention of protecting the population and the environment, establishes a list of substances and preparations to which applies certain restrictions to the commercialisation and use. The tetrachloroethane is included, and although these restrictions do not apply in research activities, this has led supply problems and a considerable increase in price. This thesis is a study of the differential solubility of polyester using non-chlorinated solvent/non-solvent mixtures, particularly phenol/water (Ph/H2O) as an alternative to the phenol/tetrachloroethane (Ph/TCE) mixture. If the results are satisfactory, it will be possible to use this technique so useful for the textile industry, with a solvent/non-chlorinated solvent mixture. The technique is optimized and applied to three different groups of polyester fibers. In each of these groups, the fibers exhibit differences in their fine structure. Results have been compared with the differential solubilty in Ph/H2O mixtures with the values obtained for these same samples with other test to determine the technical feasibility of this test. It concludes that the use of a unchlorinated phenol/water (Ph/H2O) mixture in the physico-chemical test of differential solubility is appropriate for determining the fine structure differences in the 3 groups of substrates studied and that the method is acceptable in terms of reproducibility. However, it is necessary to adjust the concentration and temperature Ph/H2O test for each group of substrates to be compared., En la fabricació de fibra de polièster primer té lloc la síntesi de les molècules que formaran el polímer, després la seva polimerització, i posteriorment la filatura i l'estiratge. Les condicions a les quals tenen lloc aquests processos, tals com la temperatura, velocitat d'estiratge o temps, tenen gran influència en l'estructura fina de la fibra resultant (cristal•linitat i orientació). Si es produeixen variacions, voluntàries o no, d'aquestes condicions, es poden produir diferències en les propietats de la fibra. Això es deu al fet que l'estructura fina de la fibra resultant pot presentar també irregularitats, és a dir, diferències de cristal•linitat i/o d'orientació. És important detectar aquestes irregularitats en l'estructura al més aviat possible, ja que les propietats afectades poden ser diverses: des d'una irregular deformabilitat del material, fins a una tintura irregular. Per aconseguir el producte tèxtil final, s'han de realitzar diversos processos. En cadascun d'aquests processos (ordit, teixidoria, blanqueig, tintura, aprests, acabats,…) tenen lloc consums tant energètics com d'aigua i productes químics, que comporten un cost econòmic. Quant més aviat es detectin les irregularitats involuntàries, menors seran les pèrdues econòmiques de l'empresa., En la fabricación de fibra de poliéster primero tiene lugar la síntesis de las moléculas que formarán el polímero, después su polimerización, y posteriormente la hilatura y el estirado. Las condiciones a las que tienen lugar estos procesos, tales como temperatura, velocidad de estirado o tiempo, tienen gran influencia en la estructura fina de la fibra resultante (cristalinidad y orientación). Si se producen variaciones, voluntarias o no, de estas condiciones, se pueden producir diferencias en las propiedades de la fibra. Ello se debe a que la estructura fina de la fibra resultante puede presentar también irregularidades, es decir, diferencias de cristalinidad y/o de orientación. Es importante detectar estas irregularidades en la estructura lo antes posible, ya que las propiedades afectadas pueden ser diversas: desde una irregular deformabilidad del material, hasta una tintura irregular. Para conseguir el producto textil final, se tienen que realizar diversos procesos. En cada uno de estos procesos (urdido, tejeduría, blanqueo, tintura, aprestos, acabados,…) tienen lugar consumos tanto energéticos como de agua y productos químicos, que conllevan un coste económico. Cuanto antes se detecten las irregularidades involuntarias, menores serán las pérdidas económicas de la empresa.