The effect of water activity on enzyme catalyzed enantioselective transesterification was studied using a solid/gas reactor. The experimental results were compared with predictions from molecular modelling. The system studied was esterification of 2-pentanol with methyl propanoate as acyl donor and lipase B from Candida antarctica as catalyst. Experimental data showed a pronounced water activity effect on both reaction rate and enantioselectivity. The enantioselectivity increased from 100 at water activity close to 0 to reach a maximum of 320 at water activity 0.2. Molecular modelling revealed how a water molecule could bind in the active site and obstruct the binding of the slow reacting enantiomer. Measurements of enantioselectivity at different water activity and temperature showed that the water molecule had a high affinity to the stereospecificity pocket of the active site with a binding energy of 9 kJ mol-1 and lost all its degrees of rotation, corresponding to an entropic energy of 37 J mol-1 K-1.