A wood preservative that increases resistance to biodegradation and fire comprises (1) 10-50 weight % sodium pentachlorophenate, (2) 20-45% borax and (3) 20-45% boric acid. The action of the carbon dioxide in air converts (1) to water-insoluble pentachlorophenol in an external layer enveloping a boron-rich inner layer. -- AATA