Green and air-dried rubber wood blocks were treated with a fluoride-and chromium-containing preservative by pressure impregnation. For comparison of treatability and improvement in durability, air-dried poplar and beech wood were included in the experiment. The liquid absorption was highest in rubber wood and lowest in beech, so was the degree of penetration. In treated rubber wood and poplar, the fluoride or chromium content dropped sharply within a few millimeters from the surface and thereafter decreased only slightly whilst in beech, it continued to drop since only its outer zone was impregnated by the preservative. Only a few of the decay test pieces were significantly attacked by one or both the rotting fungi, Coniophora puteana and Polystictus versicolor, and most of them were from the untreated inner zone of beech. The effctive level of the product used against these two decaying organisms was around 4 kg/m3, i.e. 1 kg/m3 fluoride.