Heavy metal pollution presents a major hazard to the soil environment. Studies have shown that the activities of a variety of soil enzymes are inhibited by heavy metals. However, little information is available concerning the effect of heavy metals on the activity of enzymes immobilized by different soil constituents. The main objective of this work was to investigate the effects of copper on the activity and kinetic properties of acid phosphatase both free and immobilized on two variable-charge soil clays and the minerals kaolin, goethite and manganese oxide. The effect of different forms of copper on enzyme activity was also examined. In the presence of copper chloride, the activity of free and immobilized enzymes was inhibited at copper concentrations of 0.005-0.8 mM at pH 5.0 and inhibition increased at pH 6.0. The inhibitory effect of copper chloride was greater on the enzymes bound by the two soil clays and kaolin than those by goethite and MnO2. Addition of copper chloride decreased both the Km values and the Vmax/Km ratios of free and all forms of immobilized enzymes, and showed mixed type inhibition kinetics. Comparing the effect of different forms of Cu, the residual activities of free enzyme and soil clay-enzyme and kaolin-enzyme complexes were higher when copper citrate was used than with copper chloride. The reverse was true for the enzymes immobilized on goethite and MnO2. These results indicate that the inhibition by Cu of enzymes immobilized on soil components are influenced by the properties of the adsorbent and the form of Cu, as well as pH.