The present invention relates to an impregnating agent for wood against fungi which will break down the cellulose and lignin in the wood. The impregnating agent comprises a mixture of potassium diformate and propionic- and/or acetic- and/or benzoic acid and/or a fixating agent. Said impregnating agent may comprise 0.5 5 weight % of the fixating agent selected among fatty acids, polymers and formates other than potassium diformate.