U radu je prikazana kakvoća vode jezera primjenom indeksa kvalitete vode (eng. Water Quality Index, WQI) i njihov kemijski sastav Piperovim dijagramom. Ispitivana su jezera: Vrana na otoku Cresu, Vransko jezero kod Biograda na Moru, Baćinsko jezero Crniševo i akumulacija Butoniga u razdoblju 2010.-2012. godine. U izračunu WQI korišteni su sljedeći pokazatelji: pH vrijednost, ukupna otopljena tvar (TDS), hidrogenkarbonati, ukupna tvrdoća, kloridi, sulfati, nitrati, fluoridi, kalcij, magnezij, željezo i mangan. Za izračun WQI korištene su preporuke vode za piće Svjetske zdravstvene organizacije (eng. World Health Organization, WHO) te dobiveni WQI predstavlja prikladnost voda za uporabu u vodoopskrbi.. Istraživanjem je utvrđeno da je kakvoća vode na Vranskom jezeru kod Biograda n/m bila najlošije kvalitete u 2012. godini (WQI=870), dok je najbolja kakvoća vode zabilježena na jezeru Vrana na otoku Cresu u 2012. godini (WQI=31,2). Prema Piperovom dijagramu ispitivana jezera se po kemijskom sastavu razlikuju ovisno o hidrološkom dobu, dok je Vransko jezero na Cresu ujednačene kakvoće..; The paper presents lake water quality as obtained by the application of the Water Quality Index (WQI), and its chemical composition as determined by the Piper diagram. The following lakes were investigated: Lake Vrana on the Cres island, Lake Vrana at Biograd na moru, Lake Crniševo of the Baćina Lakes, and the Butoniga reservoir in the period 2010-2012. When calculating the WQI, the following indicators were used: pH value, total dissolved solids (TDS), hydrogencarbonates, total hardness, chlorides, sulfates, nitrates, fluorides, calcium, magnesium, iron and manganese. For the WQI calculation, the drinking water recommendations of the World Health Organization (WHO) were applied, and the obtained WQI represents the suitability of water for use in water supply.. The investigation determined that the water quality in Lake Vrana at Biograd n/m was of the lowest quality in 2012 (WQI=870), while the best water quality was recorded in in 2012 in Lake Vrana on the Cres island (WQI=31.2). According to the Piper diagram, the analysed lakes differ in their composition depending on the hydrological period, while Lake Vrana on the Cres island was of a uniform quality.