An apparatus and method for purifying boron trichloride by removal of phosgene. In accordance with the invention, phosgene is photolytically decomposed into carbon monoxide and chlorine and liquid vapor phase in equilibrium with one another and in solution with the boron trichloride. This equilibrium is disturbed preferably by sparging a vapor stream that rises through the boron trichloride and is composed of the substance, such as nitrogen, to disturb the equilibrium and thereby cause the carbon monoxide and chlorine to be carried out of solution and collect in the overhead space. The decomposition product of chlorine and carbon monoxide can then be removed from the vapor space so that the carbon monoxide and chlorine does not recombine and contaminate the boron trichloride with phosgene.