Equivalent chain lengths (ECL) have been determined for methylcis-5-, 6-, 8-, 9-, 10-, 11-, 12-, 15-;trans-2-, 3-, 5-, 6-, 8-, 9-, 10-, 11-, 12-, 15-; and 17-octadecenoates and for methyl 5-, 6-, 8-, 9-, 10-, 11-, 12- and 15-octadecynoates on diethylene glycol succinate, polyphenyl ether and 100% β-cyanoethylmethylsiloxane capillary columns. Although the levels of ECL values differed, the patterns were similar for the different substrates. For methyl octadecenoates, except for 2-isomers, ECL generally increased with distance of double bond from the ester group but changed little from the 6 to the 10 position. Differences were smaller among isomerictransesters than amongcis. In methyl octadecynoates there was a greater and more regular increase of ECL occurred as the triple bond moved away from the ester group