Though lipases are frequently applied in ester synthesis, fundamental information on optimal pH or substrate concentration, can almost only be found for the reverse reaction / hydrolysis. This study demonstrates that the pHoptima of lipase-catalysed esterifications differ significantly from the optima of the hydrolysis reaction. In the esterification of n-butanol and propionic acid with lipases of Candida rugosa (CRL) and Thermomyces lanuginosa (TLL) pH-optima of 3.5 and 4.25, respectively, were found. This is about 3/4 units (CRL) and 7 units (TLL) in pH lower than optimum for hydrolysis. Enzyme activity increased with increasing concentrations of protonated acid indicating that the protonated acid rather than the deprotonated form is the substrate for esterification. The rate of esterification can be drastically increased by ensuring acid concentrations up to 1000 mmol L1 for CRL and 600 mmol L1 for TLL in the reaction system.