A net bag method has been used to study the decomposition of Salix alba leaves in five Garonne corridor sites (fast running water, slow flowing water, standing water, floodable willow stand, non floodable willow stand). Changes in the contents and the amounts of some leaf litter constituents were assessed over a 40 week period. Willow leaves decomposed rapidly and, in some cases, this could be described by an exponential model (k = 0.0040 to 0.0121/d) depending on site). Carbon content was rather stable. An absolute increase in nitrogen was observed at all sites for the first weeks. Cellulose loss was less rapid in slow flowing or standing waters and on floodable soils. The lignin fraction accumulated rapidly and was generally 50% after 40 weeks. Decomposition rates were not significantly different at aquatic and terrestrial sites. Both in waters and on soils, oxygen levels and temperature control the decomposition kinetics, especially of cellulose and lignin.