Reactive epoxides, formed as intermediates in the enzymic hydroxylation of olefinic compounds, can be trapped by their reaction with trimethylchlorosilane and detected by gas chromatography and combined gas chromatography-mass spectrometry as the trimethylsilyl derivatives of their chlorohydrin adducts. Mass spectrometric identification of the products is facilitated by the characteristic isotope ratio of chlorine. By using these derivatives, epoxides have been detected as intermediates in the metabolic conversion in the rat of allylic barbiturates to their corresponding glycols.