The decomposition rate of senescent leaves of Avicennia germinans, Laguncularia racemosa and Rhizophora mangle in their respective areas of dominance were analyzed in the estuary mangrove of the Paraiba do Sul River, Rio de Janeiro, by the method of litter bags. Results indicated that the rate of decomposition of leaves of L. racemosa (3.2 x 10(-3)) did not differ significantly from A. germinans and R. mangle (P > 0.05), but that A. germinans (5.1 x 10(-3)) exhibits a higher decomposition rate than R. mangle (2.7 x 10(-3)) (P < 0.05). These differences may be explained by different leaf traits among species (e.g. water content, concentrations of tannin, and nitrogen). The half-life calculated was 138, 216 and 257 days for A. germinans, L. racemosa and R. mangle, respectively. Considering that A. germinans has the greatest abundance in relation to R. mangle, and its kinetics of decomposition was faster, we suggest that the former species represents a major source of organic matter originated in mangroves of the estuary of Paraiba do Sul River.