ESR* studies on the role of iron in bacterial oxygenases, 3,4-dihydroxyphenylacetate 2,3-oxygenase [EC 1. 13.1.7) and catechol 1,2-oxygenase (EC 1. 13. 1. 1, pyrocatechase], demonstrated the direct participation of the iron in these enzyme reactions. 3, 4-Dihydroxyphenylacetate 2, 3-oxygenase showed no ESR signal. An ESR signal at g=4.3 appeared in the presence of substrate and oxygen, the signal disappeared after the exhaustion of oxygen. Pyrocatechase showed an ESR signal at g=4.3. It disappeared upon addition of substrate, and reappeared following the introduction of oxygen. 3, 4-Dihydroxyphenyl-acetate 2, 3-oxygenase showed ESR signals at g=2 region, Sm=1.99, 2.01, and 2.02, by the interaction with nitric oxide. The signals were not observed in the presence of substrate. Pyrocatechase showed no ESR signal with nitric oxide in the presence of substrate or dithionite. From these observations, the valency change of the bound iron during the enzyme reactions was strongly suggested. It was also suggested that the mechanism of the enzyme reaction and the state of ligands of the iron differed between these two enzymes.