The aim of the present study was to use the analysis of surface water chemistry to understand vegetation succession pathways in terrestrializing polyhumic lakes. We hypothesized that Sphagnum mire development was accompanied by a decrease in the mineral content in water. A total of 111 vegetation plots along 23 transects were analysed in 11 lakes and adjacent peat lands in the Wigry National Park (NE Poland). The vegetation of the lake-mire systems forms distinct zones: (1) nymphaeid-, bladderwort- and bryophyte-dominated aquatic vegetation; (2) sedge-dominated edge of the Sphagnumcarpet; (3) quaking, extremely poor fen with various Cyperaceae; (4) non-quaking, Eriophorum vaginatum-dominated bog-like vegetation and (5) pine woodland. Surface water corrected conductivity (ECcorr.), pH, COD-KMnO4 and Ca2+, Mg2+, Fetot. and SiO2 were measured along the transects. The environmental gradients best explaining the observed pattern were pH (with the highest values in the lake and the lowest in the bog-like vegetation) and COD-KMnO4 (showing an inverse direction). At least in some Sphagnum-mires conditions were more minerotrophic than in the lakes. The process of humic lake overgrowing by Sphagnum-mires in NE Poland results in pine woodlands on mineralised peat. The climate conditions in NE Poland, combined with evapotranspiration accelerated by encroaching trees, do not seem to support the development of ombrotrophic bogs.