The exclusion chromatography of sodium poly(styrene-sulfonate) (NaPSS) on controlled pore glass was studied at varying pH and ionic strengths. The divergence of the molecular weight calibration curves of NaPSS from those of the nonionic polymers dextran and pullulan may be ascribed to two types of electrostatic effects: Intramolecular chain expansion and polyion-substrate repulsion. Resolution of these two effects was achieved through “universal calibration” plots, which adjust for differences In chain dimensions. The difference between universal calibration plots for NaPSS and neutral polymers (which vanishes at high ionic strength) was analyzed in terms of a “repulsion volume”, Vp which reflects the diminution of the effective pore size for the polyanions due to the electrical double layer at the surface of the glass. The measured Vr at one pH and ionic strength was used to predict Its value at another set of conditions, assuming that the double-layer potential at the boundary of the repulsion volume is constant.