The effect of water activity (aW) on Candida antarctica lipase B (CALB) activity and enantioselectivity towards secondary alcohols was assessed. Experimental results for the resolution of racemic pentan-2-ol, hexan-3-ol, butan-2-ol and octan-4-ol by immobilized CALB-catalyzed acylation with methyl propanoate, were obtained by using a solid/gas reactor. Water and substrate adsorption mechanism on immobilized CALB, were then studied using moisture sorption analyzer and inverse gas chromatography, and the effective hydration state of the biocatalyst when varying aW was defined. The data showed a pronounced aW effect on both activity and enantioselectivity. If secondary alcohol follows the steric rules for being efficiently resolved, water at very low aW increased enantioselectivity by acting predominantly as an enantioselective inhibitor, making the stereospecificity pocket smaller. When increasing aW, water decreased enantioselectivity, due to an unfavourable increase of the entropic term TR-SS‡ of the differential free energy of activation. The "turning point" at which water changed from one predominant role to another would correspond to aW allowing full coverage of polar groups of the immobilized biocatalyst by water molecules.