The objective of our study was to compare leaf breakdown rates (k) and the influence of microorganisms and aquatic invertebrates on mass loss of Eichhornia crassipes (Mart.) Solms and Typha domingensis Pers in 2 reservoirs (eutrophic and oligotrophic). We hypothesized that k would be higher in eutrophic than in oligotrophic conditions because of increased microbial activity in eutrophic conditions. We collected green leaves, which we air-dried, weighed, placed in litter bags, and incubated in each reservoir. We calculated k (negative exponential model) for each species in each reservoir. We characterized initial leaf chemistry, estimated total microbial biomass (as adenosine triphosphate [ATP]) and fungal biomass (as ergosterol), and evaluated invertebrate community composition and structure. Both species decomposed faster in the eutrophic reservoir. During leaf breakdown, bacteria were more important in the eutrophic reservoir, whereas fungi were more important in the oligotrophic reservoir. Invertebrate communities differed between reservoirs, but invertebrates did not affect k in either reservoir. Our results indicate that leaf breakdown may have been accelerated by greater nutrient availability and variations in O₂ concentration and water temperature that increased microbial community metabolism in the eutrophic reservoir. Typha domingensis held nutrients in its tissues for longer than E. crassipes, and might be useful for management of nutrients in reservoirs, whereas E. crassipes decomposed rapidly and would not be useful for controlling eutrophication.