A laboratory study was conducted on the anode reaction in aluminium electrolysis prior to and during anode effect at voltages up to 12 V. The experimental methods used were cyclic voltammetry and gas chromatography, and the contents of AlF 3, CaF 2 and Al 2O 3 were varied. Three voltammetric peaks at around 2.2 V, 2.6 V and 4.3 V were observed in all low-alumina melts, except in a highly acidic melt. An additional peak at around 3.3 V was observed in Na 3AlF 6 and Na 3AlF 6 - 5 wt% CaF 2 melts. The peak currents observed in low-alumina melts were all low (< 80 mA cm -2). The peak currents of the first two peaks (at around 2.2 V and 2.6 V) increased sharply by the addition of alumina to the melts. CO was the dominant oxygen-containing gas in low-alumina melts. CO 2 was only found at low potentials (< 4 V). CF 4 generation was detected at potentials above 2.8 V. The rate of CF 4 generation was similar in the various melts, and it was in all cases low (< 40 mA cm -2). It has been shown elsewhere that alumina depletion precedes a potential shift in the positive direction, to reach the potential region for generation of CF 4. This is accompanied by a sudden drop in current, marking the onset of the anode effect.