The phase behaviour of poly(ethylene oxide) (PEG) and poly(methyl methacrylate) (PMMA) blends, a partially miscible system, and of PEO/PMMA block copolymers, all cast from acetonitrile, were determined in the presence of added NaSCN. The phase behaviour is dominated by the added salt, which forms an insoluble P(EO(3).NaSCN) complex and induces separation of PEO and PMMA in homopolymer blends. In block copolymers the PEG was either complexed with NaSCN or miscible with the PMMA block. Essentially the same phase diagram applies to the homopolymer blends and PEG-rich copolymer with added NaSCN and is identical to that previously found for mixtures of PEO and NaSCN cast from methanol. For the PMMA-rich copolymer the phase diagram is similar except that there is no evidence of PEG crystallinity, which is replaced by a miscible PEO/PMMA phase. Block copolymers in the presence of added salt were found to exhibit an additional endotherm (similar to 280 degrees C) which is attributed to a disorganization of the microphase-separated system at temperature above the melting point of the complex and is associated with a liquid-liquid phase separation. This process was found to be irreversible on cooling from the melt.