WE were searching for a model of an enzymatic reaction in the hope that the relatively simple chemistry of the model reaction might throw some light on the enzyme reaction itself. In particular, we were interested in finding a model which would duplicate some of the properties of lipases, notably their ability to catalyse the hydrolysis of lipids and the characteristic pH dependence of lipolytic activity which reaches a maximum near pH 8.5 with most lipases, and one just below pH 7 with some. Lipases being proteins, the model would have to be restricted to the functional groups available in proteins. The high potency of lipases, despite this restriction, together with the well-known chemical mechanism of ester hydrolysis, made us suspect that bifunctional catalysis1 might play a decisive part.