1. 1. NaN3 inhibits the haemin-catalysed decomposition of H2O2. The inhibition is of the competitive type. Haemin itself does not combine with azide. 2. 2. The results are in agreement with a mechanism in which the formation of two intermediate complexes between haemin and H2O2 is assumed. Inhibition occurs by a reversible combination of azide with the secondary haemin-H2O2 complex. 3. 3. The inhibition is not complete. The catalyst-substrate-inhibitor complex still possesses some residual activity. 4. 4. Increased affinity for azide in the presence of H2O2 has also been observed in catalase. The analogous behavior towards azide may indicate a basic similarity of the catalytic mechanism in the two systems.