The decomposition processes of organic matter during winter in a 3-year old soil-based constructed reed bed with lateral sub-surface water flow were evaluated by quantifying the in- and efflux of metabolic gases and by considering input-output budgets and relating these to storages within the bed. The total flux of gaseous oxygen into the bed was 5.86 g m−2 day−1 of which 2.08 gm−2day−1 was through the cavities of dead, still standing culms of Phragmites australis. The respiratory oxygen consumption of the roots and rhizomes almost perfectly balanced the oxygen influx through the culms leaving only 0.02 g O2 m−2 day−1 to be released to the surrounding soil. The macrophyte-induced rhizosphere oxygenation was therefore of no quantitative importance for aerobic BOD degradation and microbial nitrification. Organic matter was degraded aerobically by means of oxygen delivered directly from the atmosphere and anaerobically by methanogenic bacteria in the upper layers of the soil. Some nitrogen and phosphorus was retained with sludge deposition on the surface of the bed and in the inlet trench. The constructed reed bed under study did not function according to the theory of the root-zone process. The majority of the wastewater moved along preferred pathways on the surface of the bed because of low soil hydraulic conductivity. Changes in the physical design and the operational practice of constructed reed beds needed to improve and optimize performance efficiency for nutrients are discussed.