The decomposition rate of the leaves of Alnus acuminata H.B.K spp. acuminata, was studied during a two year period using the litter bag procedure. The experiment was performed in forest stands of three age group (5-10, 20- 25 and 40-45 yr) located in San Javier hills, Province of Tucumán (Argentina). A regression between the remnant percentage of dry matter and time was carried out adjusting the curve to an exponential model. The pattern of decomposition presented the most important loss of weight during the first 30 days, the rate decreasing slowly later on. Highly significant differences were observed because of the canopy closure or because the differences between the leaf chemical composition of the different age of the stands or because of both. Results are similar to those obtained by other authors working on the same genera.