We tested whether carbon loss in the form of sediment‐associated microbial respiration differed between oil sands process‐affected (OSPM) and reference wetlands. The constituents of OSPM wetlands (high salinity, conductivity and naphthenic acid concentrations) were expected to strain microbial communities and alter gas (methane and carbon dioxide) flux rates as well as sediment oxygen consumption compared to fresher, reference wetland sites. Studying 4 OSPM and 4 reference wetlands over the summers of 2009 and 2010, we found OSPM wetlands released less gas (~10 x) (methane) than reference wetlands. In 2009 we measured sediment oxygen demands (SOD) and used gas flux estimates of carbon dioxide to estimate biologically sediment oxygen consumption (BSOD). Chemical sediment oxygen demand (CSOD) was estimated by subtracting BSOD from total SOD. We found SOD rates were 2 x higher in OSPM wetlands than reference; CSOD was greater than biologically consumed oxygen for both wetland classes (~90% of total SOD). OSPM wetlands released less than reference wetlands, suggesting less microbial activity in OSPM wetlands; however, more oxygen is consumed (likely chemical oxidation) in OSPM than in reference wetlands. Carbon accrual is a necessary condition to reclaiming boreal wetlands in the Alberta Athabasca region. Less microbial activity may promote carbon accumulation (reduced respiration = less carbon loss) within OSPM wetlands but high chemical oxygen consumption may limit benthos respiration and reduce organic inputs into a wetland’s sediment organic layer.