In a controlled simple aqueous system containing Zn-goethite, where a clear particle concentration effect (Cp effect) is observed, an increase in particle concentration (Cp) causes a simultaneous decrease in adsorption reversibility and in the adsorption isotherm. At the same time, Zn adsorbed under a lower Cp condition desorbs faster (indicating more adsorption reversibility) than that under a higher Cp condition. In another controlled simple aqueous system of Cd-goethite, where no Cp effect is observed, changes in Cp does not cause discernible changes in adsorption hysteresis and in the adsorption isotherm. Little difference in desorption rate is observed for the Cd adsorbed under different Cp conditions. The experiments were designed to avoid several important sources of error. Both the Cp effect and the non-Cp effect results are qualitatively explained and quantitatively described by the MEA theory(1).