[1] Measurements of soil CO 2 efflux, litter production, and the surface litter pool biomass were made over a 1 year period in a tropical transitional forest near Sinop, Mato Grosso, Brazil with the aim of quantifying the seasonal variation in soil respiration and litter decomposition and the annual contribution of litter decomposition to soil CO 2 efflux. Average annual soil CO 2 efflux (±95% confidence interval (CI)) was 7.91 ± 1.16 g C m À2 d À1 . Soil CO 2 efflux was highest during the November-February wet season (9.15 ± 0.90 g C m À2 d À1 ) and lowest during the May-September dry season (6.19 ± 1.40 g C m À2 d À1 ), and over 60% of the variation in seasonal soil CO 2 efflux was explained by seasonal variations in soil temperature and moisture. Mass balance estimates of mean (±95% CI) decomposition rates were statistically different between the wet and dry seasons (0.66 ± 0.08 and 1.65 ± 0.10 g C m À2 d À1 , respectively), and overall, decomposition of leaf litter comprised 16% of the average annual soil respiration. Leaf litter production was higher during the dry season, and mean (±95% CI) leaf litter fall (5.6 ± 1.7 Mg ha À1