The "miscibility" and esterification in poly(styrene-co-maleic anhydride) (PSTMA)/phenoxy blends were investigated by DSC and FTIR. The blends prepared by casting exhibited a single composition-dependent but broad T-g during the first scanning. The broadness of the T-g transition range is due to the presence of microphases in the blends, which acquired some stability because of the hydrogen-bonding interactions with the continuous phase. However, the blends displayed two distinct T(g)s during the second scanning, which can be attributed to phenoxy-rich and PSTMA-rich phases dispersed one in another at a scale larger than the initial one. To investigate the effect of esterification, the samples subjected previously to two scannings have been additionally heat-treated several times between 30 and 220 degrees C and annealed each time at 220 degrees C for increasing periods of time. During the additional scannings, the two T(g)s identified during the second scanning increased with increasing annealing time but remained distinct. The fact that the fraction soluble in tetrahydrofuran decreased with increasing annealing time indicates that crosslinking due to esterification has occurred in both phases. The two phases generated after the first scanning were stabilized by the esterification reaction at the interfaces.