The phase boundaries as a function of copolymer composition have been established in one polymer/copolymer and two copolymer/copolymer blend systems involving the repeat unit methacrylonitrile. Using the critical conditions for miscibility according to the Flory-Huggins theory, and an overall blend interaction parameter (B(blend)) expressed in terms of repeat unit interactions B(i-j), values of B(S-MAN) and B(MMA-MAN) have been determined. These are compared with the corresponding acrylonitrile interactions.