The heats of solution in orthodichlorobenzene were measured as a function of temperature for both quenched and as‐received polyphenylene oxide. The curves are linear below 110°C with slopes of 0.098 cal/g°C and linear above 110°C with slopes of 0.032 cal/g°C. The inflection at 110°C is attributed to a &bgr; transition. The plot for the as‐received material is displaced vertically from that of the quenched material by 2.75 cal/g which is attributed to the fact that the as‐received material contains about 27.5% crystallinity. The major component of the heats of solution is the exothermic contribution from the solvent‐induced freeing or relaxation of the constrained molecular motions.