End-matched lumber of Picea glauca (Moench)Voss (white spruce) and Populus tremuloides Michx. (aspen poplar) timbers was treated by a thermal diffusion process in open tank treating vessels using an ammoniacal copper-arsenate wood preservative. The process proved technically feasible with respect to controlling the vapourization of ammonia from open tanks during treatment at high temperatures. Treatments of 48 hours or more on unseasoned and partially dried lumber produced net oxide retentions above that required by the Canadian Standard Association CSA-080 wood preservation standard for timber in above ground contact situations. Although preservative penetrations did not meet the penetration requirements (10 mm), of the CSA 080.2 standard for ground contact, five of the seven non-pressure charges on spruce lumber had heartwood penetrations greater than 7 mm in depth. A 24-hour treatment on air-dried spruce had penetrations equivalent to a five-hour vacuum-pressure treatment. Retention was adequate for above-ground exposure

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