Canadian green export lumber is protected against sapstain and mould by surface treatment with sapstain control products. For this purpose, coastal sawmills in British Columbia use sapstain control formulations based on didecyldimethylammonium chloride (DDAC). All Canadian green export lumber destined for Europe is also heat treated (pasteurized), in line with requirements to reduce the risk of transferring pathological nematodes to European forests. Recent experience has shown that the anti-stain performance of DDAC-treated wood is sometimes reduced after the wood has been heat treated. In the absence of research into this problem, the reasons for this remain speculative. It is possible that heat treatment modifies the interaction between the problem fungi and the wood (for example, heat treatment may clear a niche for problem fungi to become established or mobilize nutrients which stimulate fungal growth). It is also possible that heat treatment modifies the sapstain control treatment itself (Byrne, 1996). Although the chemical stability of DDAC is not affected by temperatures normally encountered during heat treatment, other factors may play a role (for example, kiln condensation may leach DDAC from the wood surface or the high humidity may affect the distribution of DDAC in the wood in such a way that the concentration of DDAC near the surface is no longer sufficient to inhibit fungal growth). The study described in this report was designed to examine the effects of heat treatment on DDAC retention and depth penetration under controlled conditions and to provide a basis for a qualitative comparison of DDAC penetration in spray-treated and dip-treated wood (Weigel and Daniels, 1995).