In the present paper, determination of isotherms of the exchange reaction between Cu(II) and clays is presented, verifying again the “cation exchange-surface precipitation” isotherm worked out formerly in our laboratory. A modified BET equation was applied to treating the experimental data and describing the new isotherm. In addition, the mechanism of the effects of organics are further discussed and verified. Based on the isotherm equations and the estimates of the average area occupied by a single molecule on the surface, we put forward a new hypothesis which differs from the current one of “organic film”. In the present paper, determination of isotherms of the exchange reaction between Cu (II) and clays is presented, verifying again the “cation exchange-surface precipitation ” isotherm worked out formerly in our laboratory. A modified BET equation was applied to treating the experimental data and describing the new isotherm. In addition to the isotherm equations and the estimates of the average area occupied by a single molecule on the surface, we put forward a new hypothesis which differs from the current one of “organic film ”.