Decomposition of plant litter in water influences metabolic processes, so its investigation has a major priority. In Hungary at the banks of lakes and rivers, Salix and Populus trees are the most common species, which contribute to the growth of nutrient load in the water. In an experiment at Lake Balaton and Kis-Balaton Wetland System between 16 November 2017 to and 3 June 2018, the decomposition rates of willow (Salix sp.) and poplar (Populus sp.) leaves were investigated with the commonly used litterbag technique in water. Total nitrogen and phosphorus content of biomass samples were measured at the beginning and end of the experiment. The decomposition rates were classified to the medium decomposition category in the large mesh size for willow and the leaves in the Lake Balaton, while other devices have slow decomposition rates. In the area of Lake Balaton and Kis-Balaton Wetland System a significant decrease of total phosphorus content was determined in plant samples. But the total nitrogen content increased at both sample sites in both plant species. The publication is supported by the EFOP-3.6.3-VEKOP-16-2017-00008 project. The project is co-financed by the European Union and the European Social Fund.