Membrane inlet mass spectrometry (MIMS) was used to monitor continuously and simultaneously the concentrations of dissolved gases (O 2 , CO 2 , CH 4 ), in the willow root zone at the laboratory scale, and within the treatment bed of a willow vegetation filter treating leachate at a landfill site in mid Wales. These results demonstrate that willows are able to release oxygen into the root zone which accumulates during daylight. Diurnal cycles of oxygen, carbon dioxide, and methane were observed, whereby CO 2 and CH 4 varied reciprocally in relation to O 2 . The intensity of these cycles appeared to be related to light intensity and temperature. Oxygen was shown to fluctuate between completely anaerobic and fully aerated (300μM), between day and night in sunny conditions.