Leaf decomposition rates of nine types of litter, ranging in lignin content from 1.04 to 25.71%, were measured at three wetlands with different inundation regimes and related to internal and external factors. In these sites (located in the alluvial floodplain along at transect perpendicular to the main channel of the Paraná River), 180 litter bags were incubated under water. To test the importance of initial nitrogen, lignin, phosphorus, or L:N content of the litter in predicting decay rates, we analyzed the relationship between leaf quality and dry weight remaining (at 30 days and at the end of the incubation) within each wetland and under uniform environmental conditions. To elucidate the effects of environmental factors (especially hydroperiod) on decomposition processes, we compared the decay coefficient of one of these species (Eichhornia crassipes) across the three wetlands. Our results strongly suggest that under uniform environmental conditions, decomposition rates were best explained by a single internal factor related to litter quality. We found two different situations. 1) In the marsh with riverine forest, where decomposition was rapid and the half-life of the litter ranged from 9.6 to 63 days, the initial nitrogen content of leaves was a good predictor of decay rates both at 30 days and at the end of the incubation. 2) In the oxbow lake and palm swamp forest, where decomposition was lower and the half-life varied between 31.5 and 219 days, the L:N explains the variability on dry weight remaining at the end of the incubation. The decay coefficient of E. crassipes across sites was significantly related to NO₃⁻ concentration of water. We concluded that both environmental variables (i.e., the fluvial quotient of connectivity and the number of days in high water) and leaf litter quality (i.e., nitrogen content and L:N) are important factors regulating leaf litter decomposition in Paraná River floodplain wetlands.