The present invention relates to a preservative formulation for use in treating wood or other cellulosic materials, said formulation comprising: at least one biocidal metal compound at least one preservative and an aqueous carrier, wherein the preservative formulation has an uptake in the sapwood of less than about 220 l/m3 yet achieves substantially full penetration of the sapwood. Preferably, the preservative formulation is copper/tebuconazole in a ratio of about 25/1 w/w and the wood is around 25 mm in thickness—for instance, decking timber.
