The miscibility criteria of a copolyester of bisphenol-A with terephthalic and isophthalic acids (50/50) (PAr) and poly (2,6-di methyl-1,4-phenylene oxide) (PPO) was investigated at 30 C in chloroform and 70 C in dioxane dilute solutions by viscometric analysis. The intrinsic viscosity and viscometric parameters of this blend system were determined for several PAr/PPO mixtures in compositions such as 80/20, 60/40, 50/50, 40/60, 20/80 in chloroform and 75/25, 50/50 and 25/75 in dioxane solutions. The miscibility criteria on the basis of the sign of Delta b, Delta b', Delta[eta], alpha and beta which are the difference between their experimental and ideal values were calculated by applying equations proposed by Krigbaum-Wall, Catsiff-Hewett, Garcia et al., Sun et al. and Jiang-Han. The data showed that the prepared blends were immiscible in the studied compositions at 30 and 70 degrees C. However, variation of the values of miscibility criteria indicates that the blend tends to be miscible by increasing temperature and PPO content in blend.