Epoxides of methyl esters of elaidic and oleic acids were allowed to react with methanol, ethanol, n-propanol, iso-propanol and n-butanol, in the presence of iodine, to give the corresponding alkoxyhydroxy methyl esters. Ethyl elaidate epoxide gave a hydroxymethoxy methyl ester when treated with boron trifluoride in methanol but the ethyl ester group was not attacked with iodine as catalyst. Mass spectra of the alkoxyhydroxy esters contained strong peaks which demonstrated the location in the chain of the original epoxide ring. Iodine also catalysed the addition of water to methyl elaidate or oleate, giving erythro- and threo-9,10-dihydroxyoctadecanoates, respectively. The alkoxyhydroxy esters were quantitatively converted to t-butyldimethylsilyl ethers by reaction with t-butyldimethylchlorosilane/imidazole/dimethylformamide reagent at 100°C but the dihydroxyoctadecanoates were not completely derivatised. Mass spectra of all the t-butyldimethylsilyl ether derivatives contained intense fragments allowing the molecular weights and the positions of the ether functions to be easily determined.