An Al target has been sputtered in a planar magnetron system using argon and argon–oxygen mixtures at pressures from 0.1 to 2 Pa. The characteristics of both dc and rf discharges have been studied. In the dc case, the current is given by a relation of the form KVn where V is the applied voltage and K and n are pressure-dependent parameters. Values of n up to 9 were obtained at the higher pressures but n decreased at lower pressures. For the rf discharge, the target self-bias voltage, Vsb, at a given power decreased with decreasing pressure and was related to the average rf power by a relationship of the form CVsb/(Vsb−V) where V was approximately 1000 V and C dereased from 900 to 500 W with increasing argon pressure. The deposition rate of Al increased linearly with average dc and rf power and there was good agreement between the values for the same power; the rate was 0.93 μm/min at the maximum power of 2.5 kW. When sufficient oxygen was added, the target surface was oxidized and the deposition rate of Al2O3 was only 5% of the Al rate. This is about half the ration predicted for sputtering yields and can be accounted for by a secondary electron emission coefficient, due to ion bombardment, of between 0.4 and 1.2. This is much higher than the value for Al and accounts for a sharp decrease in Vsb when oxide forms on the target surface. At an average power of 400 W at a pressure of 1 Pa, Vsb is 380 V for the Al case and 190 V for the oxidized target.