Sulfonated polystyrene is frequently studied as a model polyelectrolyte because of its narrow molecular weight distribution, corresponding to that of the precursor, anionically polymerized polystyrene. Frontal analysis continuous capillary electrophoresis (FACCE) was carried out for polystyrenesulfonate (PSS) samples from various commercial sources. Whereas some of the electropherograms appeared as abrupt and discontinuous, indicating a single uniform mobility, others were sigmoidal, corresponding to a distribution of mobilities. This distribution must reflect sample heterogeneity, possibly due in part to the molecular weight distribution (MWD) for the low-MW samples, but only attributable to chemical heterogeneity for the higher-MW samples. Size-exclusion chromatography (SEC) was used to establish the possibility of some MWD contribution to the distribution of mobilities for the low-MW PSSs, but not for the high-MW ones. Because a high-MW sample with a broad MWD prepared by polymerization of styrenesulfonate did not show a mobility distribution, the only plausible explanation for those distributions is compositional heterogeneity, i.e., chain-to-chain variations in the degree of sulfonation.