To help delineate the important phenomena which should be included in fundamental models of catalyst impregnation, the cause of the retardation of Pt adsorption over oxide supports, at pH ranges in which the oxides dissolve, has been studied with a series of experiments in which the possible causes for the retardation are isolated. These effects are oxide dissolution, hypothesized by some to cause a loss of adsorption sites, and high ionic strength, which according to a simple electric double layer theory causes "double layer compression" and an effective decrease in the adsorption equilibrium constant. The latter effect is independent of pH. Two adsorption systems were studied : platinum chloride adsorption over alumina was measured at low pH, while platinum amine adsorption was measured over silica at high pH. The results from both sets of experiments are the same : high ionic strength is a sufficient condition for the retardation of adsorption. Oxide dissolution appears to be a parallel process that effects adsorption only indirectly through an ionic strength increase from high levels of dissolution.