Positron annihilation lifetime measurements were performed on amorphous styrene-maleic anhydride and styrene-acrylonitrile copolymers of different compositions. Glass transition temperatures of the copolymers were determined by differential scanning calorimetry and densities and specific volumes were obtained with a gas pycnometer. The average free-volume hole size measured by the positron lifetime technique was almost constant with composition in styrene-acrylonitrile, whereas in styrene-maleic anhydride it first increased up to 28% maleic anhydride content and then decreased at higher concentrations. This is caused by differences in copolymer constitution, which changes from random to alternating with increasing concentration of maleic anhydride, leading to differences in the packing of the polymer chains. The results also show a clear decrease in the orthopositronium yield (I-3) with increasing maleic anhydride or acrylonitrile content of the copolymer, which is interpreted as inhibition of positronium formation by these components. An equation previously used to describe the inhibiting effect of scavengers in non-polar solvents and of halogenated compounds added to polycarbonate could be fitted to the experimental values.