Quality of waters depends on several factors, including the amount of nutrients, toxic materials, different physical and chemical reactions and the living organisms as well. The decreasing quality in fresh water bodies can lead to several disasters, and in order to prevent this, the Water Framework Directive has the aim of reaching good ecological conditions in surface waters. The water quality of Lake Balaton has dropped due to the drainage of Kis-Balaton Wetland, the incoming alluvium of River Zala was deposited in Keszthely-Bay without filtering from the wetland. The KBWPS was designed to reconstruct this filtering function and it has been successful. The aim of our study is related to this reconstruction: to determine the effects of Kis-Balaton Wetland on the water quality of Lake Balaton. Our samples were taken from both water bodies and the main chemical variables of the water samples (NO, NIL', SO..PO,' and CI) were analysed in laboratory. The results correlated to former researches in the topic and the positive effect of the KBWPS was shown, especially in the difference of phosphate concentration of Lake Balaton and Kis-Balaton Wetland. KBWPS has a great effect on the oligotrophication of Lake Balaton. 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.