Substituted chiral thiophene 1-oxides and their cycloadducts of variable enantiopurity have been isolated as products of dioxygenase-catalysed sulfoxidation of the corresponding thiophenes using intact cells of Pseudomonas putida; thermal racemization (ΔG^‡= 25.1 kcal mol^–1) of the enantiopure metabolite (1R)-2-methylbenzo[b]thiophene 1-oxide has been observed.