Ethylene carbonate, a cyclic organic carbonate widely used industrially, is toxic when metabolically converted to ethylene glycol. A rat liver enzyme active in catalyzing the ring opening has been purified to electrophoretic homogeneity and found to be active in the hydrolysis of ethylene, vinylene, and propylene carbonates to CO2 and the respective glycols. Neither thiocarbonates nor open chain carbonates served as substrate nor did a variety of esters, lactams, lactones, and related heterocycles. The enzyme was active, however, with imides and appears to be identical to rat liver imidase. The identification was confirmed by copurification of enzyme activities, by similarities in the pattern of inhibition, and by the reactivity with a polyclonal antibody directed against the enzyme purified here.