Disclosed is a method of making phosphorous acid or hypophosphorous acid by reacting hydrogen chloride with a sodium phosphite or a sodium hypophosphite, respectively, in the presence of water to precipitate sodium chloride crystals and form the acid. The acid is separated from the sodium chloride crystals and can be passed through an anion exchange column that is preferably loaded with phosphite or hypophosphite ions, respectively, to remove residual chloride ions.