Abstract The diethyl esters of cis- and trans-epoxysuccinic acid were synthesised and examined. The compounds were stable for short periods at pH 5·0, as determined by gas-liquid chromatography. Both compounds were non-toxic to mice, possibly being detoxified in the liver by conjugation with glutathione. Both esters alkylate γ-( 4-nitrobenzyl)pyridine, which may indicate that they are carcinogenic. Compounds with related structures possessed no alkylating properties. Gas-liquid chromatography has not yet indicated the presence of the epoxy esters in extracts of distilled drinks. The possibility that nitrosamines, lactones and some esters might be biologically active is discussed.