The water activities (a(w)) of 13 salt hydrate pairs were determined from vapor pressure measurements; a(w) values for a subset were also estimated from a study of water transfer to isopropylether. The application of salt hydrates as water buffers was investigated in two models: (i) effect of hydration on the initial rate of subtilisincatalyzed transesterification of the nitrophenol ester of CBZ-alanine with butanol; and (ii) effect of hydrates on the equilibrium concentrations of reactants in the esterification of dodecanol and decanoic acid, catalyzed by lipase. Transfer of ions from salt to enzyme particles was also demonstrated. The implications of the results for the successful use of salt hydrates as water buffers are discussed. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 367-374, 1997.