Our study examines the effect of seasonal rains on soil organic C dynamics in a tropical deciduous forest ecosystem in Western Mexico. At the end of the wet season, an accumulation of labile nutrient forms developed and was maintained during the dry season. This accumulation enhances microbial activity in the first rains of the wet season. For example, the litter samples of the dry season had a higher C and N mineralization than those of the wet season. Similarly, the January soil samples had higher C mineralization than October soil samples (55 and 34 μg C g-1 d-1, respectively). These results suggest that the quality of C is strongly affected by the seasonality of rains, which in turn influences microbial activity. This seasonality also influences nutrient redistribution between soil aggregate fractions. Chemical changes across seasons suggest that soil organic matter associated with macro-aggregates represents the main source of energy for microbial activity at the beginning of the wet season, while micro-aggregates protect the labile nutrient forms during the growing season.