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The various theories previously put forward to explain and predict the constitution of polyanionic phosphate glasses are assessed. It is shown that the most satisfactory is that in which the deviation from the ideal or Flory distribution of chain lengths is explained in terms of the enthalpies of each of the anion disproportionation reactions by which the distribution is obtained from a monodisperse liquid. The extension of this theory to other systems is considered, but the main aim of the experimental work has been to verify some of the postulates and determine the factors controlling the deviation from the Flory distribution. The transition temperatures of glasses of previously determined constitution were measured by differential thermal analysis and used to calculate the enthalpies of the reorganisation reactions among the polyanions. The devitrification characteristics were also examined qualitatively. It was found that the glass transition temperature was approximately three fifths of the melting point of the nearest crystalline component but was slightly affected by the liquidus temperature. Its small variation with the mean chain length of the system is evidence that the anions are able to reorganise quite readily so that the melt is in internal chemical equilibrium at all temperatures above its transition temperature. The dependence of the chain equilibria on the basicity, the fictive temperature of the glass, and the cation were examined. The heat of the disproportionation reactions whereby a distributed series of chains is formed from a monodisperse system increases slightly as the mean chain length decreases. The effect of the fictive temperature on the distribution could not be determined satisfactorily, but the results obtained are compatible with the assumption of an enthalpy of disproportionation. The constitution in a lead phosphate glass was determined. It showed that the narrowness of the distribution is dependent on the heat of formation of the phosphates from their constituent oxides and not on the ratio of the cation size to its charge, as suggested from the changes in coulombic energy when disproportionation occurs. It is suggested that since the coulombic interaction theory is useful in explaining the distributions in mixed cation and fluorinated phosphate glasses, the effect of electronegativity differences on the degree of ionic bonding and the coulombic interactions should be considered.