Abstract: A method for controlling the magnesium content of molten aluminum includes the steps of injecting a halogen gas into the molten aluminum, sampling a portion of the resulting gases evolved from the molten aluminum, detecting the presence of constituents in the sampled gases and/or characteristics of the sampled gases indicative of the imminent evolution of unreacted halogen gas, and adjusting the rate of halogen gas injection to approach the point where unreacted halogen gas is about to be evolved. In the preferred embodiment, the halogen gas is chlorine; the detected constituents are gaseous hydrogen chloride (CHl) and aluminum chlorhydrate compounds (Al.sub.2 Cl.sub.x .multidot.(OH).sub.6-x); and the detected characteristic is the opacity of the sampled gases. The invention includes a technique for adjusting the rate of halogen gas injection so as to approach the optimal rate. The invention also includes (1) a sensor for detecting the content of HCl and Al.sub.2 Cl.sub.x .multidot.(OH).sub.