Sorption isotherms, spectroscopic studies, and chromatographic retention data were used to elucidate the mechanisms involved in the separation of strong electrolytes on polyacrylamide gels. For the most part Bio-Gel P-2 was investigated but some results on the dextran gel Sephadex G-10 are also discussed. On P-2 the primary mechanism is not sieving or exclusion on the basis of size but rather a weak physical sorption of the cations to the gel and probably a hydrogen-bond interaction between the anions and the amide hydrogens. Molecules and ions which are not sorbed on the gel are partly excluded from a fraction of the internal solvent volume which is involved in hydration of the gel matrix and is firmly bound to it. When chromatographing solutions at low sample concentrations on some polyacrylamide gels,Knfalls off drastically. This effect is more pronounced in Sephadex than in Bio-Gel and it has been interpreted as resulting from a Donnan exclusion of anions which arises from a small number of anionic groups attached to the gel matrix. Equations relating elution to the various mechanisms are developed.