Reclamation of sites affected by oil sands mining in the Athabasca Oil Sands Region (AOSR) targets the construction of new fen watersheds, which are dominant wetland types in the region. The aquifers of slopes that supply water to the fen watershed are formed with tailings sands containing residual oil sands process-affected water (OSPW) contaminants, whose effects on peat microbial community function are poorly explored. To understand the effect of potential OSPW contamination on microbial communities typical to the range of peatlands in the AOSR, we measured microbial functional characteristics (overall substrate-induced respiration (SIR) and catabolic evenness) and tested the effect of short-term in-vitro exposure to OSPW in peat samples from three representative fen types (treed rich fen, poor fen, and hypersaline fen) within the AOSR at the start (early May) and middle (late June) of the growing season. Overall, our results suggest that short-term exposure to OSPW has negligible impact on peat aerobic microbial activity, and that time of growing season and site physicochemical characteristics are the primary control on microbial potential activity. Further studies are necessary to assess the effects of OSPW contaminants on microbial-driven processes in the medium and long terms, under anaerobic conditions, which dominate in peatlands.