Kinetic descriptions of the effect of clay minerals on the activity of enzymes are needed for a better understanding of the interaction of enzymes and clays in soil. We used a model clay-enzyme system to study the kinetics of inhibition f acid phosphatase and pyrophosphatase activities in homogenized corn roots by Na-homoionic montmorillonite, illite, and kaolinite clays. The adherence of the inhibition data to the partial noncompetitive kinetic model in the presence of montmorillonite or illite suggested that corn-root acid phosphatase and pyrophosphatase bind to the clay to form a complex whose maximum rate of activity (Vmax) is smaller by a factor of β, but whoseKmvalues remain unchanged. The β values determined for the inhibition of acid phosphatase by montmorillonite and illite were 0.33 and 0.53, respectively. The corresponding β values for inhibition of pyrophosphatase were 0.42 and 0.32. The inhibition of acid phosphatase and pyrophosphatase activities by kaolinite could be explained by a partial competitive inhibition model. TheVmaxvalues measured remained the same in the presence of kaolinite, but the apparentKmvalues were increased by a factor of α. The α values determined for acid phosphatase and pyrophosphatase were 4.6 and 3.2, respectively. The partial noncompetitive and partial competitive models predict a limit to the amount of inhibition observed in the presence of the various clay minerals. This indicates that a clay-enzyme complex may form whose activity is reduced but not totally eliminated and suggests that the formation of a clay-enzyme complex may be a mechanism by which a certain proportion of enzyme activity in soil may become stabilized. Shaking of the samples during incubation did not change the type of kinetics observed, but did change the values of the kinetic constants.