Previous work on lipase catalyzed esterification of glycerol with fatty acids in w/o microemulsions gave monoglyceride as the main reaction product. Only traces of triglyceride were obtained. We show here that the reason triglycerides are not formed in these systems is that the diglyceride formed in the oil-water interfacial zone partitions into the continuous hydrocarbon domain. It is then inaccessible to the enzyme. The yield of triglyceride is increased by using longer hydrocarbons as the oil component. These have poorer solvency for mono- and diglycerides. When the hydrocarbon chain length exceeds that of the glyceride acyl groups, triglyceride is the main reaction product.