The miscibility and phase behaviour of alternating copolymers of the type [CHPh-CHX-CO-Y-CO-CH]n (X is Me, OEt, Osec-Bu; Y is oxygen, N-Me, N-Ph) in blends with PPO and sulphonylated PPO (SPPO) have been studied by differential scanning calorimetry, and by visual observation of film clarity. Alternating copolymers of beta-methylstyrene with maleic anhydrice (MAn) or with N-substituted maleimide (RMI) were found to be immiscible with PPO and SPPO copolymers in all proportions. The same was found for the blends of alternating copolymers of PhCH=CHOR with MAn or with NMeMI in blends with PPO and SPPO copolymers. It was also found that blends of poly(PhCH-CH-OR-alt-NPhMI) (R is Et, n-Bu) are miscible with SPPO having a degree of sulphonylation higher than 60 mol%, but they were immiscible with SPPO of a low degree of sulphonylation. The high-temperature phase separation in the miscible blends is a function of the degree of sulphonylation of SPPO and of the blend composition.