Arsenic (As) oxidation during full-scale aeration and rapid filtration was investigated. In the supernatant water, As(III) showed no apparent oxidation and meagre adsorption to HFO was observed. However, in the top of the filter bed rapid As(III) oxidation occurred and the subsequently formed As(V) was efficiently adsorbed. Although MnO2 could potentially oxidise As(III) it was found that As(III) oxidation was inhibited in the presence of Mn(II) and Fe(II). During ripening of virgin sand filters, As(III) oxidation was complete after 21 days of operation, before Mn(II) and NH+ 4 oxidation. Therefore, it may be concluded that formation of MnO2 could not have been responsible for As(III) oxidation, strongly suggesting that this is a microbial process. This process could develop amongst common groundwater bacteria and mineral precipitates, directly leading to an increased As removal in the filter bed.