This paper deals with the esterification kinetics of oleic acid with ethanol using both Amberlyst-15 and lipase from Aspergillus niger (Novozym 435) as catalysts. The reaction was carried out in a stirred glass reactor between 30 - 80°C under conditions with negligible external and internal transport resistances. Indeed, from the plot of (1/-r) vs (1/m) ((-r) = reaction rate; m = catalyst loading), the ratio of external to the combined pore diffusion and surface reaction resistances was small (1.325 x 10^-2) suggesting that the surface reaction was kinetically-controlled. The rate has a first-order dependency on oleic acid with an activation energy of 70.2 kJ mo^l-1. However, enzymatic esterification did not follow Arrhenius behaviour. Maximum rate was obtained at about 45°C and decreased monotonically thereafter. Even so, enzymatic rates were about 3 to 10 times higher than those obtained from the ion-exchange resin. Although literature studies have indicated no agreement in the optimum water content, painstaking examination of the effect of water concentration in the reaction mixture has revealed, for the first time, the existence of multiple peaks in the rate-water content behaviour over a wider range than previously investigated. This explains the apparent discord in earlier investigations. The oscillatory kinetics appears to be a characteristic of the dual role of water as product inhibitor and enzyme activator. This peculiar attribute leads to sinusoidal revivification of the enzyme activity. An interactive model for this phenomenon is presented in the full paper.