Carbon sources, microbial community production and respiration were determined in nine northeastern Alberta wetlands subject to oil sands mining. A stable isotope mixing model estimated the proportion of carbon sources (primary production, sediment and petroleum) assimilated by microbes. The proportions of petroleum-derived carbon from oil sands process material (OSPM)-affected wetlands ranged from 62-97%. Bacterioplankton production was quantified by monitoring 3H-leucine incorporation into bacterial proteins. Production and methanogenesis were inhibited by sulphate and/or salinity. Amending wetland sediments with topsoil, a reclamation strategy, did not affect bacterial production or stimulate decomposition. Unvegetated wetland sediments were small net exporters of C and do not appear to be on a trajectory to becoming net sinks in these early stages of development. Overall, microbial functional processes in OSPM- affected wetlands are markedly different from processes in reference-constructed wetlands of equivalent age and do not correspond to those in a mature natural wetland.