We report results on the effects of porewater pH and transients in porewater flow on the deposition and mobilization of colloid-sized clay particles within unsaturated sand columns. The deposition rates of illite under steady-flow conditions were essentially independent of pH, while the deposition rates of kaolinite nearly doubled as the pH decreased from 7.4 to 4.6. Mobilization of kaolinite colloids was slow or negligible under steady-flow conditions; however, transients in porewater flow induced rapid colloid release. A model that accounts for rate-limited deposition reactions and that links colloid mobilization to variations in moisture content and porewater velocity describes the effluent colloid concentrations measured during the steady-flow and transient-flow stages of the column experiments. On the basis of these results we infer that the effects of pH on clay-colloid deposition depend on the mineralogy of the clay colloids and that perturbations in flow are critical in mobilizing clay colloids within the vadose zone.