Poly(styrene-co-methacrylic acid) (PSMA) mid poly(styrene-co-4-vinylpyridine) (PS4VP) of different compositions were prepared and characterized. The phase behavior of these copolymers as binary PSMA/PS4VP mixtures or with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) as PPO/PSMA or PPO/PS4VP and PPO/PSMA/PS4VP ternary blends was investigated by differential scanning calorimetry (DSC). This study showed that PPO was miscible with PS4VP containing up to 15 mol % 4-vinylpyridine (4VP) but immiscible with PS4VP-30 (where the number following the hyphen refers to the percentage 4VP in the polymer) and PSMA-20 (where the number following the by hen refers to the percentage methacrylic acid in the polymer) over the entire composition range. To examine the morphology of the immiscible blends, scanning electron microscopy was used. Because of the hydrogen-bonding specific interactions that occurred between the carboxylic groups of PSMA and the pyridine groups of PS4VP, chloroform solutions of PSMA-20 and PS4VP-15 formed interpolymer complexes. The obtained glass-transition temperatures (T-g's) of the PSMA-20/PS4VP-15 complexes were found to be higher than those calculated from the additivity rule. Although, depending on the content of 4VP, the shape of the T-g of the PPO/PS4VP blends changed from concave to S-shaped in the case of the miscible blends, two T-g were observed with each PPO/PS4VP-30 and PPO/PS4VP-40 blend. The thermal stability of the PSMA-20/PS4VP-15 interpolymer complexes was studied by thermogravimetry. On the basis of the obtained results, the phase behavior of the ternary PPO/PSMA-20/PS4VP-15 blends was investigated by DSC. (c) 2007 Wiley Periodicals, Inc.