A cytochrome P-450 LM2 reconstituted system mediated expoxidations of small terminal alkenes to the corresponding alkyloxiranes. This oxidation was accompanied by a stereoselective proton exchange of the E proton. Propene was oxidized in D2O to trans-3-deuterio-2-methyloxirane, and E-1-deuteriopropene was epoxidized in H2O to methyloxirane. Thus, oxidation occurred with complete exchange. The epoxidation of 1-butene in D2O generated a mixture of 86% ethyloxirane and 14% E-3-deuterio-2-ethyloxirane. Reaction of E-1-deuterio-1-butene produced 10-30% ethyloxirane and 70-90% trans-3-deuterio-2-ethyloxirane. Larger terminal alkenes in the homologous series did not undergo such an exchange. A mechanism involving formation of organoiron porphyrin intermediates is proposed. Stereoselective deprotonation by a basic amino acid residue producing an iron carbene complex is proposed as the mechanism of exchange with water.