The composition and monomer unit sequence distributions of copolymers of styrene with citraconic (alpha-methylmaleic) anhydride (CA) prepared in methyl ethyl ketone at 50.0 +/- 0.1-degrees-C have been determined over a range of comonomer feed mole fractions using C-13 n.m.r. spectroscopy. The monomer units in these copolymers were found to display an increasingly strong tendency to alternate as the mole fraction of CA in the feed increased. The terminal, penultimate, complex-participation and complex-dissociation models were each tested for applicability to the mechanism of copolymerization for this comonomer system by applying reactivity ratio related test functions to the experimental sequence distribution data, enabling a direct visual assessment of the applicability of each model to be made. On the basis of these test functions, the terminal, penultimate and complex-dissociation models were each found to provide inadequate descriptions of this copolymerization system.