The hydration of cis-β-methylstyrene oxide, cis-2,3-octene oxide, and their 18O-enriched forms by epoxide hydrase of rat liver microsomes has been investigated. Both cis epoxides underwent quantitative enzymatic hydration yielding exclusively the corresponding threo diols, indicating that complete stereochemical inversion at a single oxirane carbon had occurred. Mass spectral analysis of diols formed enzymatically from the 18O enriched epoxides indicated they were formed with great regioselectivity, 89% and 85% of the 18O being located at the benzylic carbon of the styrene diol and at C-3 of the octane diol, respectively. © 1978.