Despite visual similarities between marshes and moderate-rich fens in the southern part of the western boreal forest of Canada, there are ecological and functional differences between the two wetland classes. Western boreal marshes have received little attention and are often difficult to distinguish from moderate-rich fens in appearance, especially when both are located at the edges of ponds and small lakes. We examined species assemblages, net above-ground production, decomposition, nutrient cycling, and environmental characteristics in five wetlands, all with approximately 1 m of peat. Based on TWINSPAN and DCCA, both wetland classes contain Carex aquatilis, but marshes were characterized by Carex utriculata, Carex atherodes, Lemna minor, Typha latifolia, and no mosses. Fens were characterized by Carex lasiocarpa and various species of brown mosses, and overall, they were more variable in species composition. There were also functional differences between the classes, with net above-ground production (especially vascular plant production) greater in marshes. Decomposition rates were higher in marshes, with Carex mass losses of 86% compared to 73% in the fens after 506 days. Comparisons of our data with previous years also indicate that high water levels promote decomposition and suggest that differences in the decomposition rates between marshes and fens may ultimately be controlled by hydrologic conditions and not surface-water nutrient dynamics. Nutrient dynamics and concentrations differed between wetland classes. Concentrations of phosphorus were higher in marsh plants and lower in the peat soil than in fens. Nitrogen concentrations were higher in marsh peat than fen peat. Based on the N:P quotients of the dominant vegetation, plants in both wetlands were more limited by available N than P. Surface-water concentrations of all nutrients were higher in marshes than fens, although even the fens can be considered eutrophic, with total phosphorus concentrations averaging approximately 100 μg L⁻¹. Surface-water levels were generally higher in the marshes, and 90% of the variation in environmental variables between the two peatland classes could be explained by surface-water level, alkalinity, chloride, ammonium, and potassium. Thus, we found that marshes and moderate-rich fens have characteristic species assemblages and that these two wetland classes can be distinguished based on environmental variables and ecosystem functions, despite the strong visual similarities between some fens and marshes.