Fatty acid ester derivatives and a process for their production from unsaturated fatty acids are disclosed. The process comprises: a) reacting an unsaturated fatty acid or an ester thereof having one or more sites of unsaturation, with an epoxidation reagent to form a fatty acid epoxide wherein at least one of the sites of unsaturation of the fatty acid or fatty acid ester is converted to an oxirane ring; and b) reacting the fatty acid epoxide produced in a) with a carboxylic acid to form a hydroxy fatty acid ester derivative wherein the oxirane ring is opened and converted to a hydroxy ester comprising a hydroxyl group at one carbon of the opened oxirane ring and an ester of the carboxylic acid at the other carbon of the opened oxirane ring.