The interaction between poly(methymethacrylate) (PMMA) and poly(vinyl chloride) (PVC) has been studied in dilute urea solutions of dimethylformamide (DMF) at 28℃ using a dilute solution viscometry method. The results show that the polymer mixtures are compatible in DMF solution in the absence of urea. The influence of urea addition on the degree of compatibility of the polymer mixtures has been studied in terms of the compatibility parameters (Abm and A[r]m). It was found that the compatibility of the polymer mixtures is decreased with increasing urea addition, passing through a minimum at 0.5 M urea.