The phenomenon of swelling resulting in volume transitions of cross-linked poly(4-vinylpyridine) and poly(N,N-dimethylacrylamide) resins in organic solvents induced by bis(2-ethylhexyl)dithiophosphoric and bis(2,4,4-trimethylpentyl)dithiophosphinic acids is demonstrated. Observed volume transitions in resins are explained by the dithioacid-polymer complexation analogous to the surfactant-polymer association in aqueous media.