The presence of naturally occurring volatile halohydrocarbons in marine organisms, seawater, and the upper atmosphere has prompted a serach for their biosynthetic origin. An earlier report documented the preparation of an enzyme extract from a marine algae which catalyzed the formation of dibromomethane, tribromomethane, and 1-bromopentane from 3-oxooctanoic acid. This report did not establish a pathway nor did it examine potential intermediates involved in the synthesis of the halometabolites (Theiler, R., Cook, J., Hager, L., & Siuda, J. (1978) Science (Washington, D.C.) 202, 1094-1096). This paper shows that an extract of the green marine algae, Penicillus capitatus, which contains a potent bromoperoxidase activity, is capable of catalyzing the incorporation of bromide ion into organic combination in the presence of 3-oxooctanoic acid. By use of gas chromatography and mass spectroscopy, it has been possible to identify tribromomethane, 1-bromo-2-heptanone, 1,1-dibromo-2-heptanone, and 1,1,1-tribromo-2-heptanone as products of this reaction. The properties of the enzymatically synthesized products have been compared to authentic compounds and found to be identical. The mono- and dibromoheptanones can be utilized as precursors for the enzymatic formation of tribromoheptanone, but the final hydrolysis of the tribromoheptanone to bromoform appears to be a nonenzymatic reaction with the P. capitatus extracts.