Surfactants which adsorb at solid/water interfaces may induce the coadsorption of various charged or uncharged molecules. In the present investigation, weak acids (1-naphthoic, Caminosalicylate, 8-nitrophenol, 2,4,6-trichlorophenol) are coadsorbed on silica using cetylpyridinium chloride and weak bases (4-toluidine, 1-naphthylamine, 1,10-phenanthroline) are coadsorbed on alumina with sodium octylbenzenesulfonate. The effect of pH changes on the coadsorption effect for both types of systems has been studied in a pH range between 3 and 9. The surfactant concentration was kept constant corresponding to the isotherm region of a low surface coverage. Solute partition coefficients Pads (for. the undissociated forms) and binding constants Kg (for the completely dissociated forms) have been determined. It is shown that in all cases the solute coadsorption goes through a maximum at a pH value equal to the pK of the acids or of the bases. It is argued that the initial pH changes induces the dissociation of the solutes and a favorable interaction with the adsorbed surfactant ions. However as the pH further changes and the silica and alumina surfaces become increasingly ionized, these ionic sites are responsible of a repulsion effect on the dissociated solute species, hence the coadsorption maximum. A simple thermodynamic model which depends only upon chemical equilibria and the corresponding constants P-ads, K-B, and pK fits well the experimental data provided Kg is assumed to be a function of the solution proton activity.