Disclosed is a composition and method for preserving wood and construction materials made of wood. The composition comprises more than about 65% by weight of at least one difficulty volatile organic-chemical solvent having a flash point higher than about 30.degree. C. and of between about 0.35% by weight and 10.0% by weight of a specific fungicidal agent which is soluble, or at least partly soluble, in the organic-chemical solvent. The specific fungicidal agent is a mixture of N,N-dimethyl-N'-phenyl-N'-(fluoro dichloro methylthio) sulfamide and N,N-dimethyl-N'-(p-tolyl)-N'-(fluoro dichloro methylthio) sulfamide, in the proportion, by weight, of between about 25:10 and 10:30 and preferably between about 15:10 and 10:15. If desired, between about 0.1% by weight and 7.0% by weight of a soluble insecticidal agent may be added to the composition.