The anode reaction and related secondary reactions in aluminum electrolysis have been studied in a small laboratory cell where the total amounts of anode gas could be analyzed. The primary anode product is found to be , at least at current densities above about 0.05 amp/cm2. Assuming that is formed with 100% current efficiency, a gas volume deficit of about 4% is actually observed. This deficit increases with increasing ratio in the electrolyte and with decreasing interpolar distance, and it is most likely due to a reduction of to carbon by dissolved aluminum in the electrolyte. At normal current densities does not react with the anode carbon to form . It can, however, react with the “carbon froth” formed by disintegration of the anode. A reaction between and the anode carbon is found to occur only at very low current densities, below 0.1–0.05 amp/cm2. A simple theory for the mechanism of the anode reaction is proposed.