The enantioselective reduction of racemic sulfoxides by dimethyl sulfoxide reductases from Rhodobacter capsulatus, Escherichia coli, Proteus mirabilis and Proteus vulgaris was investigated. Purified dimethyl sulfoxide reductase from Rhodobacter capsulatus catalysed the selective removal of (S)-methyl p-tolyl sulfoxide from a racemic mixture of methyl p-tolyl sulfoxide and resulted in an 88% recovery of enantiomerically pure (R)-methyl p-tolyl sulfoxide. Rhodobacter capsulatus was shown to be able to grow photoheterotrophically in the presence of certain chiral sulfoxides under conditions where a sulfoxide is needed as an electron sink. Whole cells of Rhodobacter capsulatus were shown to catalyse the enantioselective reduction of methyl p-tolyl sulfoxide, ethyl 2-pyridyl sulfoxide, methylthiomethyl methyl sulfoxide and methoxymethyl phenyl sulfoxide. Similarly, whole cells of Escherichia coli, Proteus mirabilis and Proteus vulgaris reduced these sulfoxides but with opposite enantioselectivity.