A continuous spectrophotometric assay based on the differences in the ultraviolet spectra of trans-stilbene oxide (TSO) and its reaction product 1,2-diphenyl-1,2-ethanediol is described for the measurement of mammalian cytosolic epoxide hydrolase activity. Rates of TSO hydration determined by this method were similar to those obtained by independent analytical methods, and the assay is rapid, reproducible and relatively free from interference.