Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH4and CO2, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH4and CO2to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH4production, oxic conditions during period 2 did not result in a lower potential CH4production rate during period 3, although production was delayed ∼1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH4, oxic conditions during period 2 resulted in zero production of CH4during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH4production, the oxic period had a negligible effect on anoxic CO2production during period 3, in surface as well as deep peat. In both surface and deep peat, CO2production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO2production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.