We examined controls on mineralization of carbon to methane (CH4) and carbon dioxide (CO2) inSphagnum(moss)-dominated peatland ecosystems by transplanting surface (5 cm deep) and subsurface (40 cm deep) peat samples reciprocally among three sites for periods ranging from 4 to 25 months. The sites were Big Run Bog in West Virginia, USA, Bog Lake Bog in Minnesota, USA, and Bog 307 in Ontario, Canada. Immediately upon retrieval, we incubated the peat samples in the laboratory at 12 and 22°C under both anoxic and oxic conditions to estimate rates of carbon mineralization. Transplanting affected surface peat more than subsurface peat. Peat incubated within Bog Lake Bog in Minnesota had the highest rates of CH4production, regardless of origin, whereas transplanting did not affect rates of CO2production measured concomitantly. Peat that originated in Big Run Bog in West Virginia generally maintained higher rates of CH4production and CO2production than peat from the other two sites after incubation in the field. The temperature dependence (Q10) of CH4production and CO2production varied among transplant sites, but not among peat origins, suggesting physiological adaptations of microbial communities to local environmental conditions. Differences in organic matter quality of the peat, particularly lignin chemistry, helped explain the results: (a) CH4production correlated with fresher lignin derived fromCarexsedges, and (b) CO2production correlated with woody lignin. We concluded that, although both site conditions (climate, nutrient status, and microbial communities) and organic matter quality influence carbon mineralization in peat, interactive effects occur and may differ depending on peat temperature. Moreover, CH4production and CO2production respond differently to environmental regulators.