The swelling equilibrium of cross-linked poly(acryloyl-L-proline alkyl esters) (poly(A-ProOR), where R is methyl, ethyl, or propyl) was studied as a function of temperature in aqueous solutions of surfactants. In pure aqueous solution the poly(A-ProOMe) gel showed inverse volume phase transition at around 14 degrees C, poly(A-ProOEt) gel showed inverse volume phase transition at 2 degrees C, and the most hydrophobic gel, poly(A-ProOPr), did not show any phase transition. The effect of adding such different surfactants as sodium dodecyl sulfate (SDS, anionic), cetyltrimethylammonium chloride (CTAC, cationic), dimethyl laurylbetaine (DALB, zwitterionic), and octaethylene glycol monodecyl ether (DOE, nonionic), on the phase transition was investigated. At the low surfactant concentration the following picture emerged : Addition of the anionic or cationic surfactant to the solution raised the transition temperature as well as the swelling. Zwitterionic surfactant had no effect on the transition temperature, but the volume transition became sharper. The nonionic surfactant did not affect either the transition temperature or the volume change. With the increase of the surfactant concentration however, the effects became more complex. The changes in swelling behavior of the gels in the presence of surfactants result from the association of surfactants with the polymers and formation of mixed micelles. Our results confirm that the main driving force for this association is the hydrophobic interaction between nonionic gels and surfactants, and depends on the hydrophobicity of the gel, the nature of the surfactant, and its concentration in the solution.