Various maleimide-containing copolymers have been studied with respect to their miscibility behaviour towards poly(methyl methacrylate) (PMMA) by differential scanning calorimetry (d.s.c.). Poly(styrene-co-N-maleimide)s (NMIs) with maleimide contents between 8 and 21 wt% were shown to be completely miscible with PMMA, whereas a blend containing 45 wt% maleimide in the NMI was not miscible with PMMA. In contrast to the miscible blend NMI-21/PMMA, a NMI-21-MMA polymer containing 21 wt% maleimide, 40 wt% methyl methacrylate, 35 wt% styrene and 4 wt% maleic anhydride proved to be immiscible with PMMA. By means of d.s.c. and Fourier transform infra-red spectroscopy measurements on the polymer blends and their model compounds, it was possible to account for the miscibility behaviour in terms of competing equilibria and concentration effects.