A process for protecting wood against ligniperdous insects, termites, ligniperdous fungi, soft rot, and mold fungi, which comprises o-boric acid/propylene oxide adducts having a molar ratio of acid to propylene oxide in the range of 1: 1 to 1:5, respectively, said adducts being formed by reacting o-boric acid and propylene oxide either in an aprotic solvent at 100° to 160°C in the absence of a catalyst at 70° to 120°C in a closed vessel in an aprotic solvent and in the presence of a catalytic amount of a base or a Lewis acid at a pressure of said propylene oxide of 4 to 9 bars, and thereafter distilling off said solvent. -- AATA