The miscibility criteria of a copolyester of bisphenol-A with terephthalic and isophthalic acids (50/50) (PAr) and polystyrene (PS) was investigated at 25 degrees C in dilute solutions by viscometric analysis. The intrinsic viscosity and viscometric parameters of this blend system were determined for several PAr/PS mixtures in compositions such as 80/20, 60/40, 40/60, 20/80 in chloroform solutions at several concentrations. The miscibility criteria on the basis of the sign of Delta b, Delta b and Delta[eta] which are the difference between their experimental and ideal values were calculated by applying theoretical equations proposed by Krigbaum-Wall, Catsiff-Hewett and Garcia et al. The parameters alpha and beta were also evaluated. The data obtained from the viscometry studies showed that the prepared blends were immiscible in all the studied composition range.