The reactivities and reaction rates of the thiol-ene coupling reaction of 2-ethyl-(hydroxymethyl)-1,3-propanediol trimercapto acetate and 2-ethyl-(hydroxymethyl)-1,3-propanediol trimercapto propionate with two common unsaturated fatty acid methyl esters (methyl oleate and methyl linoleate) were evaluated. The reactions were monitored with real-time IR and H-1 NMR, which both showed that the mercapto acetate was more reactive than the mercapto propionate. Both thiols were more prone to add to the monounsaturated methyl oleate than to methyl linoleate, which contained two unconjugated double bonds. According to bond energy calculations, the thiol hydrogen of mercapto acetate was somewhat more difficult to abstract than the hydrogen of mercapto propionate. Consequently, the formed S-C bond in the acetate case was stronger than in the propionate case, and so the equilibrium was more shifted toward the addition products. The real-time IR measurements also showed that the cis unsaturation in methyl oleate isomerized much more quickly than that in methyl linoleate, and this also had an impact on the overall addition rate of the thiols because a trans unsaturation was more reactive than a cis unsaturation. The higher isomerization rates in the oleate systems, compared with those of the linoleate systems, was suggested to be due to a more restricted rotation along the C-C bond of the reacted unsaturation in linoleate. This study showed the importance of trans unsaturations in obtaining reasonable reaction rates in thiol-ene reactions with fatty acid derivatives. (C) 2004 Wiley Periodicals, Inc.