A laboratory study was performed on the effects of pure preparations of vegetable tannins on the nitrification of ammonium sulphate in soil. Purified preparations of wattle and of chestnut-oak tannins used at concentrations of 0.50, 1.00, and 2.00% (w/w) showed a fairly strong inhibition on the formation of nitrate only during the first two weeks' incubation. Similar tannin preparations used at 0.125% (w/w) concentration had practically no influence on nitrification. Generally, the wattle tannin had a slightly stronger inhibitory effect than the chestnut-oak mixture.