The activity of mushroom tyrosinase catalyzing the conversion of o-diphenols to o-quinones in organic solvents has been investigated with addition of different amounts of water and at various temperatures. The results have shown that tyrosinase possesses high activity in organic media,the enzyme activity is mainly controlled by the water activity in the reaction system,and it can increase with the increase of reaction temperature as long as the amount of water bound to the enzyme molecule is properly controlled. Salt hydrates are effective for such control in nearly non-aqueous conditions.