Use of micronized zinc compounds in wood preservation treatments containing micronized copper compounds significantly increases the efficacy of the treatment with respect to that of the copper compounds alone. Micronized copper compounds used at a retention in the range of from 0.005 to 1 pounds per cubic foot (pcf) expressed as CuO equivalent, and zinc compounds are used at a retention in the range of from 0.005 to 1 pcf expressed as ZnO equivalent, the increase in wood preservation efficacy over similar retentions of copper compounds alone can be seen in as little as 24 months.