The enthalpy relaxation of miscible blends of styrene-maleic anhydride (SMA) copolymers and poly(methyl methacrylate) (PMMA) has been studied, and the enthalpic data modelled using the Cowie-Ferguson (CF) semi-empirical approach. The blend behaviour in each case was not predictable from the concentration of each component. For the blend with the lower MA content copolymer (SMA2/PMMA), the enthalpy lost by the fully relaxed glass was greater than for either component, whereas the higher MA content copolymer blend (SMA4/PMMA) aged in a manner very similar to PMMA itself. In each case, the blends relaxed more slowly than their components and the distribution of relaxation times was broader. This implies that each blend has a lower free volume than its components; however, this conflicts with the enthalpic data for SMA2/PMMA. Reasons for this apparent contradiction are given. It is likely that concentration fluctuations are present in the SMA4/PMMA blend, suggested by a broad transition range at T,, which may explain its slower relaxation behaviour.