The ionisation state of the enzyme and the water activity (alpha (w)) of the system are two important factors which affect catalytic activity in low-water organic solvents. Here we investigate the interaction between effects of alpha (w) and the ionisation state of subtilisin Carlsberg, controlled by an organo-soluble dendrimer buffer. It was shown that alpha (w) does not have a major effect on the relationship between rate of transesterification in toluene and buffer ratio, [sodium salt]/[acid]. The weak dependence on alpha (w) (between 0.54 and 0.85) probably reflects compensating effects on ionisation equilibria for the buffer and the carboxyl groups on the enzyme.