The enthalpy relaxation of styrene-maleic anhydride (SMA) copolymers, at various ageing temperatures (T-a) below T-g and for different ageing times (t(a)), is described. Values of enthalpy lost (Delta H(t(a),T-a)) were calculated from experimental data, plotted against log(10)(t(a)) and modelled using the Cowie-Ferguson (CF) semi-empirical approach to give a set of values for three adjustable parameters, Delta H-infinity(T-a), log(10)(t(c)) and beta. These define the relaxation process, which was found to be sensitive to copolymer composition in a non-linear fashion. Increasing MA content initially causes a faster relaxation with a smaller overall enthalpy loss; further increases in MA levels lead to slower changes to a final state of lower equilibrium enthalpy. All experiments indicate that the enthalpy lost by the fully relaxed glass (Delta H-infinity(T-a)) is less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (Delta H-max(T-a)), and that the final enthalpy value is dependent on copolymer composition. The implications of this are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.