There was studied the solid-state enzyme reaction of specific substrate N-succinyl-L-phenylalanine-p-nitroanilide hydrolysis in contact with alpha-chymotrypsin (Cht). It is shown that product yield is dependent on hydration of the protein. The product is formed only when the relative pressure of water vapours (p/ps) was higher than certain magnitude (p/ps) crit; which depends on the amount of the salt in the sample: the higher the salt concentration the lesser the (p/ps) crit value. It is suggested that the counter-ions may interact with some of primary hydration sites of the Cht molecule. Because of that for the formation of the active Cht conformation is enough to bind lesser number of water molecules than in salt-free samples. But in the presence of salt excess in Cht sample it is necessary to bind of the protein surface at least yields to 80 mol H2O/mol Cht.