An impregnation process, which is efficient in the use of both energy and treating materials and which may be applied to both permeable and dense woods, comprises contacting the wood with a vaporised chlorinated aliphatic solvent at a temperature of 50-125 degrees C, at above atmospheric pressure for a time sufficient to preheat the wood to approximately the temperature of the solvent; immersing the wood in a solution of at least one wood treating agent (preservative, fire retardant or dye) in the same chlorinated solvent at a similar temperature and pressure for 0.1-10 hrs; removing the wood, and reducing the pressure to cause the solvent to evaporate. -- AATA