Isothermal phase diagrams for semicrystalline poly(ethylene-co-vinyl alcohol) (EVAL) using two nonsolvents (water and X-propanol) serving asa cosolvent system have been determined. The locations of the liquid-liquid demixing and the crystallization-induced gelation in the phase diagrams at 60 degrees C were identified. Attempts were made to correlate the experimental findings with predictions on the basis of the modified Flory-Huggins theory for ternary ablutions using three binary interaction parameters (chi(ij)) and one ternary interaction parameter (chi(T)). No good agreement was found between the theoretical predictions and experimentally obtained data if the ternary interaction parameter was neglected. But an improvement can be obtained when a ternary interaction parameter is considered. The optimum value has been found to be approximately -1.7 + 0.5(phi 2) + 1.0(phi 3). Therefore, it is necessary to introduce the ternary interaction parameter when the binary parameters provide an inadequate description of phase behavior, but in most systems studied so far the-ternary interaction parameter is ignored.