Decomposition of aquatic macrophytes in water influences metabolic processes, so its investigation has a major priority. In an experiment at Lake Balaton and Kis-Balaton Wetland between 22 September 2017 to and 17 November 2017, the decomposition rates of Myriophyllum spitatum and Ceratophyllum demersum were investigated with the commonly used litter bag technique in water. We used two mesh size of litter bags (large - ø = 3 mm and small - ø = 900 μm). The aquatic macrophytes were classified to the fast decomposition category in Lake Balaton and Kis-Balaton Wetland.