The nitrogen (N) supply is limited in many peatlands, and the mineralization of organic matter represents a major source of N to plants. The goal of this paper is to establish the relationship between decomposition rates, mineralization rates, and the N concentration of peat along a peatland gradient in four peatlands in western, boreal Canada. The four peatlands differ physically, chemically, and floristically and include a bog, a poor fen, and two moderate-rich fens, one wooded and the other open. The fens are enriched by ground water inputs, which we hypothesized led to higher concentrations of N in peat, faster decomposition of litter, and higher N mineralization rates. Net N mineralization was virtually identical to net ammonification over a 2-y period, both measured using an in situ incubation technique. Net daily mineralization rates increased from the bog to the open, moderate-rich fen, with intermediate values in the poor fen and moderate-rich wooded fen. This increasing trend along the bog–open, moderate-rich fen peatland gradient was mirrored for mineralization rates on a temporal and spatial basis. Virtually no nitrification was detected in any of the sites. Estimated aboveground integrated ecosystem decomposition rates ranged from 17%·y⁻¹ in the bog to 31%·y⁻¹ in the wooded, moderate-rich fen. Decomposition rates were significantly correlated with the mean daily net N mineralization rate and with total N in the upper 20 cm of peat. The net mineralization rate also was positively correlated with the total N content of the peat in 1991. The total N content of the peat increased from 5.8 mg·g⁻¹ in the bog to 11.5 mg·g⁻¹ in the open, moderate-rich fen, with poor fen and moderate-rich wooded fen values intermediate. The total C content of the peat ranged from 370 to 400 mg·g⁻¹ and was not significantly different among the four peatlands. TC:TN quotients for the peat cores were 68 in the bog and 36–38 in the fens, correlating negatively with net ammonification rates. Based on our data, the processes of mineralization of N and decomposition are tightly linked processes, both of which were higher in the fens than in the bog in our four western continental peatlands. Moreover, both processes are more affected by the quality of the organic matter, as indicated by the TC:TN quotients of the surface peat, than by surface water chemistry variables.