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This thesis describes a theoretical and experimental investigation of the adiabatic flow of evaporating fluids in pipes. The theory is developed from the assumptions usually made in the subject of fluid flow, and is tested by experiments with water and also with freon 12. Prom a review of previous papers relating to the subject two main points emerge. (1) An evaporating fluid, depending on the conditions, may conform to any one of a variety of flow mechanisms. (2) The liquid and vapour components may flow with different mean velocities. Appropriate theory is developed for three flow forms which are known to occur and which probably cover the majority of practical cases. These modes of flow are characterised as (a) annular, (b) separated and (c) frothing. In the first two forms relative motion between phases occurs, but in the last is prevented by a liquid network maintained by surface tension forces. The theory for annular flow shows that the relative velocity factor (ratio of mean vapour to mean liquid velocity) is a function of the thermodynamic properties of the fluid and also of the interface velocity ratio (ratio of moan velocity at interface to mean liquid velocity). This latter is the only empirical quantity influencing the relative velocity factor: its value must be near to, but not less than unity. In separated flow the relative velocity factor is also affected by the wall shearing force acting on the vapour phase. Since there is no relative motion between phases with frothing flow, standard thermodynamic theory is applicant provided surface energy may bo neglected. Critical outlet conditions readily occur with evaporating flow because of the high rates of volume increase with pressure drop. The equations which express the relative velocity factor throughout an expansion also provide the criterion for the prediction of critical outlet conditions. For annular and separated flows these are identical. Critical pressure ratios for frothing flow are much smaller than for a corresponding flow with relative motion. Critical outlet temperature and pressure charts for water and for freon 12 have been prepared and are given in the text. The main water tests were made on a tube of 0.1285 bore, a few preliminary tests being carried out on a tube of 060" bore. By measuring fluid momentum at the tube outlet an experimental determination was obtained of the relative velocity factors. Temperature and pressure measurements were made throughout the expansion and the technique adopted for those are fully described. Visual evidence of the flow form was obtained by photographing the fluid as it left the tube. Both observation and analysis confirm that the flow form adopted by the evaporating water is of the annular type. The theoretical and measured values of relative velocity factors and critical cutlet conditions are in good agreement. Experimental data on critical outlet conditions obtained previously by Burnell on pipes of 1/2 to 11/2 bore, also show agreement with the theory. Friction factors associated with the annular flow of evaporating water were investigated and a method of correlation is proposed. The wall shearing force is found to be a function of liquid velocity and density, the friction factor being dependent on a Reynold's number. The freon 12 experiments were carried out on a small refrigerator test unit of a standard type, tests being made on tubes of .042" and .060" bore. Precautions were taken to prevent frosting on the capillary tube. Critical outlet data showed conclusively that the flow form in this case was of the frothing type. This had been assumed by some previous investigators but had never been proved. The fact that freon 12 adopts the frothing flow form is consistent with the general theoretical development. The question of friction factors for frothing flow is discussed but no attempt at a complete investigation could be made because of the extensive nature of the experimental work required. The factors affecting the occurrence of the various flow mechanisms are given qualitative discussion.