Laboratory experiments were conducted to measure the activity of phosphomonoesterases in various zones of a geological outcrop. The outcrop contained an Entisol and partially weathered carbonaceous shales of Paleozoic age with differentiating zones rich in pyrite and goethite. Goethite is the most commonly occurring iron oxide in soils, and pyrite is often found in reconstructed soils of coal surface mines. The activity of acid phosphatase was significantly elevated in goethite‐rich zones compared to zones where pyrite was present. Alkaline phosphatase activity was reduced significantly in the presence of goethite but was not affected by pyrite. Experiments with pure pyrite and synthetic goethite substantiated the results obtained from the outcrop samples. The enhancing effect of goethite at lower pH values is probably due to chemisorption and removal of mineralized P, which increases the mineralization of organic P. At higher pH values goethite surfaces are negatively charged and repel phosphate anions. The exact mechanism for the inhibitory effect of pyrite on acid phosphatase is not known.