The biological oxidation of several tributyltin derivatives, by a cytochrome P-450 dependent monooxygenase enzyme system with reduced nicotinamideadeninedinucleotidephosphate as the essential cofactor, produced carbon-hydroxylated compounds identified as alpha-, beta-, gamma- and delta-hydroxybutyldibutyltin derivatives. The hydroxylation pattern and the lack of oxidative cleavage of tin-carbon bonds strongly suggest a free radical rather than an oxenoid mechanism, while the predominance of beta-carbon-hydroxylation further implies some role of the tin-carbon sigma electrons in directing the site of hydroxylation.