The miscibility of different polymer blends was studied with dynamic mechanical thermal analysis (DMTA) in conjunction with differential scanning calorimetry (DSC). The blends were prepared by melt mixing polymers having similar glass transitions such as polyglutarimide (PGI), styrene-co-maleic anhydride random copolymers (SMA), and polystyrene (PS). In PGI/SMA blends, there is only one glass transition detected with DMTA. In the case of PGI/SMA14 blends, the single glass transition temperature is due to their full miscibility. However, PGI/SMA8 blends are immiscible throughout the whole composition range as was verified by optical observation (opaque appearance), DSC, and scanning electron microscopy. The observation of only one glass transition by DMTA was attributed to weak interactions that take place between the two polymers, leading to partial mutual solubility and bringing the slightly different T-g temperatures of the pure polymers even closer. In SMA8/SMA14 blends, there are two glass transitions detected with DSC as well as with DMTA, indicating that the two copolymers are immiscible. However, in all compositions, shifts of the glass transitions were observed with DMTA, which is evidence of partial miscibility. The observation of immiscibility was easiest in PS/SMA blends by both techniques, because of the bigger difference in glass transition temperatures of the initial polymers. (C) 2004 Wiley Periodicals, Inc.