Abstract Ricinoleic acid can give four diastereoisomeric 9,10,12-trihydroxystearic acids. Each of these, when treated with acidic methanol, gives, mainly or entirely, one isomer of methyl 9,12-epoxy-10-hydroxystearate from which the corresponding 10-methoxy and 10-oxo esters are prepared. Structural identification is based mainly on mass spectroscopy. 9-Hydroxyoctadec- cis-12-enoic acid gives a similar series of 9,12,13-trihydroxystearic acids which yield 9,12-epoxy-13-hydroxy (methoxy, oxo)stearates. The mechanism and stereochemistry of the cyclisation reaction are discussed.