The measurement of the molecular weight averages and the molecular weight distribution (MWD) of many step-growth polymers is complicated due to the presence of cyclic oligomers formed during polymerization. If size exclusion chromatography (SEC) is used to determine the MWD, the cyclic oligomers are generally only partly resolved from the linear polymer, and hence distort the measured linear MWD. Further, the cyclic oligomers require a different calibration curve from the linear species and hence, in general, their molecular weights are not accurately measured. In order to clarify the effect of cyclic species on the measured MWD, a model of the SEC separation of step-growth polymers with cyclic species was developed. In this article, this model is described and used to illustrate aspects of the characterization of these polymers using both conventional SEC and multi-detector SEC. The results from the model are used to develop methods for estimating the MWD of the linear polymer and to determine the weight fraction of cyclic species. The results of the model are compared with experimental data for nylon 6, nylon 6,6 and poly(ethylene terephthalate).