To obtain a better understanding of the enzymatic properties of soils from tropical and subtropical areas polluted with heavy metals, the effect of zinc on the activities and kinetics of acid phosphatase immobilized on variable-charge soil constituents was investigated. Both free and immobilized enzymes were inhibited significantly in the presence of 0.1 to 4 mM Zn (as ZnCl2), and the inhibition increased with an increase of pH from 5.0 to 6.0. The enzymes immobilized on the Latosol clay and kaolin mineral were as sensitive as free (i.e. soluble) enzymes to Zn toxicity. The inhibitory effect of Zn was reduced when acid phosphatase was immobilized on goethite. Various inhibition patterns by Zn were observed for free and immobilized acid phosphatase at different pH values. Zinc showed uncompetitive inhibition for the goethite-enzyme complex at pH 5.0, 5.5 and 6.0. The inhibition of free enzyme and Latosol clay-enzyme complex by Zn changed from an uncompetitive type at pH 5.0 and 5.5 to a mixed type at pH 6.0. However, the inhibition model of the kaolin-enzyme complex was mixed type at the three pH levels. Variation of inhibition model seems to be related to the degree of deactivation of the enzyme by the Zn. Zinc addition also increased the affinity of free and immobilized enzymes for the substrate (i.e. reduced Km values). The high affinity of goethite-enzyme complex for the substrate induced a substrate inhibition at pH 5.0, which was enhanced by the presence of Zn.