In the boreal mixed forest, stand composition generally changes from deciduous to mixed to coniferous stands during post-disturbance succession. Our objective was to determine the influence of forest composition on the quality of soil nitrogen and carbon as determined by C and N mineralization during the course of a long-term in vitro incubation (282 days). Three stand types (Populus tremuloides, Betula papyrifera, and conifers (mixture of Abies balsamea and Picea glauca )), of two ages (50 and 124 years since fire) growing on two soil types (clay and till) were considered. Expressed on an organic C basis, our results showed a greater N mineralization for deciduous than coniferous stands, both in the mineral soil and the forest floor, a higher N mineralization in the mineral soil of older stands as compared with young ones, and in till than in clay soils. Mineralization of C was higher in the forest floor of clay soils as compared to till soils while the opposite was found in the mineral soil. It was also higher in both soil layers of older sites. The observed differences in N and C mineralization among stand types, stand ages and deposits appeared to be due to differences in the most labile fraction of soil organic matter because these differences were observed within 100 days of incubation. The ratio of C mineralized-N mineralized was greater in coniferous than deciduous soils in both soil layers, indicating a lesser quality of organic matter under coniferous stands. Despite significant differences among the above-listed factors for N and C mineralization on an organic C basis, the pool size of mineralized (or mineralizable) N and C was generally not significantly different between the different forest and soil types because of an inverse relationship between quality and quantity of soil organic matter. Correlation and multiple regression analysis indicated that clay content (negatively), C-N ratio (negatively), available P as well as exchangeable Mn were related to the mineralization of N.