Abstract: The unwanted or unreacted decaborane gas that results from a boronizing operation using decaborane which is solid at ordinary temperatures but which is heated to a gaseous phase for injection into a vacuum vessel is passed through a metal tube packed with the granules of an alkaline oxidizing agent consisting of potassium permanganate and potassium hydroxide, whereby the decaborane gas is oxidized to solid and gaseous stable substances including manganese compounds and hydrogen. This provides an effective method for removing boron from the boron-containing exhaust gases that result from the coating of the inner surfaces of vacuum vessels in nuclear fusion reactors with an evaporated boron film.