A process for the purification of by-product sulfuric acid resulting from the synthesis of boron trifluoride, wherein the ratio of fluorine to boron is three or more, which process comprises entraining the by-product acid in an inert gas, the process being characterized in that before entrainment the anhydrous sulfuric acid is diluted with water in a proportion of from about one to about ten percent by weight of water with respect to the acid to be treated, the temperature being maintained above 90.degree. C., the sulfuric acid obtained containing less than 0.003% fluorine and less than 0.005% boron, so that the quality is substantially equivalent to a sulfuric acid directly obtained by the oxidation of sulfur.