An isobutene-forming activity was reconstituted with cytochrome P450rm and cytochrome P450 reductase purified from Rhodotorula minuta. The nonionic detergent. Emulgen 911, present in the preparation of purified P450rm, inhibited the reconstitution. Bovine serum albumin enhanced the activity of the reconstituted system. Branching of the beta carbon of the substrate carboxylic acid was important for formation of isobutene. In a comparative study with isovalerate and 3-deuterio-3-methylbutanoate, a very large isotopic effect (kH/kD = 14) was observed. This result indicates that formation of isobutene might be initiated by abstraction of hydrogen from the beta carbon of isovalerate and might be followed by decarboxylation.