The adsorption of nonionic and anionic polyacrylamides in porous media has been studied as a function of substrate hydrophobicity. The adsorption of nonionic polyacrylamide was found to increase with substrate hydrophobicity, being lower on quartzitic sand particles (400 mu g/m(2)) than on the same particles rendered hydrophobic by treatment with hexamethyldisilazane, a short-chain silane (490 mu g/m(2)), or octadecyltrichlorosilane (C18), a highly reactive, long-chain silane (720 mu g/m(2)). The same trend was observed with other siliceous substrates and their Gig-treated counterparts. When the C18-treated sand particles were in turn rendered hydrophilic by adsorption of sodium dodecyl sulfate or polyethylene glycol, then polyacrylamide adsorption almost vanished. These effects are interpreted in terms of a competition between substrate/monomer and substrate/solvent (water) interactions. The adsorption of anionic poly acrylamides (acrylamide-acrylic acid copolymers) on quartzitic and C18-treated sand particles has also been measured for different solution ionic strengths, pH’s, and polymer ionic (acrylic acid) compositions and distributions along the chain. As with nonionic polyacrylamide, adsorption was significant and increased with substrate hydrophobicity when electrostatic long-range interactions were not predominant, e.g., at low pH or high salinity. For significant electrostatic interactions, however, the inverse trends were observed : adsorption levels were low and decreased from quartzitic to C18-treated sands. The origins of such repulsion between polyelectrolyte and hydrophobic substrate are discussed.