The average molar masses of styrene-maleic anhydride (SMA) copolymers, calculated from data obtained by size exclusion chromatography (s.e.c.), are often lower (by a factor of two) or higher (by a factor of eight) than the values expected from light scattering and viscometry. However, the polystyrene samples which are used as reference materials exhibit no such deviations. Therefore, the exclusion mechanism during the elution of SMA is significantly interfered with by certain adsorption and repulsion mechanisms, with these being dependent on the set of columns used, the comonomer content and the molar mass. This interference can be attributed to the existence of active polar sites on the packing and to partial ring opening of the maleic anhydride ring, thus resulting in the formation of a dicarboxylic acid species. Ring opening might also be followed by protolysis. This anomalous behaviour can be suppressed by the addition of 5 wt% acetic acid to the solvent used, i.e. tetrahydrofuran (THF). The presence of the acetic acid does not influence the intrinsic viscosity. Thus the coil dimensions (hydrodynamic volume) do not change and the universal calibration remains valid.