The invention relates to agents for the protective treatment of wood against decay and insect infestation, which contain at least the following ingredients: a) between 10 and 90 parts coaltar oil, b) between 2 and 20 parts of a quaternary ammonium compound with biocide action, and c) between 10 and 90 parts water. The agents provided for by the invention are easily prepared stable emulsions characterized, among other things, by good penetrating power, significantly reduced odour of the treated wood, absence of undesirable coaltar concentrations on the surface, markedly reduced bleeding of the treated wood during summer temperatures, significantly greater effectiveness despite a lower coaltar oil content and lower toxicity than pure coaltar oil.
