The main ions and mineralization are one of the most stable and conservative indicators of water chemical composition, which are put at the basis of several classifications of natural waters. The waters from hydrogen carbonate class, group of calcium are most favourable sources for drinking water supply. The investigations on the Lower Prut water chemical composition have been carried out from March 2014 to April 2015 by using accepted in hydrochemistry methods and ISO standards. It was determined that in investigated waters of the Prut River the hydrogencarbonate and carbonate ions predominate on sulphate and chloride ions, thus, according to the classification of Aleokin (1970) they are assigned to hydrogencarbonate class. Diapason of these ions on Gotesti-Giurgiulesti sector was placed between 172.7 mg/l and 247.1 mg/l, which correspond to multiannual dynamics. Along the entire river course the average concentration of sulphates made up 75 mg/l. The highest values were recorded in March 2015, and the lowest - in September 2014. The content of chloride ions in Prut waters not exceeded 40.0 mg/l, with one exception – in Mach 2015, downstream to Giurgiulesti, when it reached 48.5 mg/l. Sodium and potassium ions varied from 28.3 mg/l to 118.5 mg/l, at all stations the highest values being registered in the beginning of the spring. Calcium ions dominated among cations; their concentration was relatively stable, in most of cases around 50 mg/l and only in spring (March) it increased up to 75-80 mg/l. It was registered no increase of content of calcium ions along the Prut course, due to the influence of Danube waters, which penetrates into the Lower Prut when the flow of the last one is low. The concentration of magnesium ions ranged 14.6 - 34.6 mg/l, revealing the highest values at Cahul and Giurgiulesti stations in March 2015. The Prut water hardness varied from 3.85 mg-echiv/l (Gotesti, Cahul, September 2014) to 6.60 mg-echiv/l (Giurgiulesti, March 2015). As mentioned above, all main ions put in evidence the highest concentrations in March (both of 2014 and 2015), and as result, the highest values of water mineralization were also registered in the beginning of spring – in average 684 mg/l, for all investigated station. And opposite, all main ions had the lowest values in the beginning of autumn and consequently, the water mineralization had the lowest values – in average 372 mg/l. This is explained by the presence of floods in March – usually the heaviest during the year, accompanied by intense surface run-off and the dilution of river water in September by rainfalls. According to the Regulation on environment quality requirements for the surface waters (2013), in most of cases the investigated Prut waters, according to their main ions, referred to the quality classes I-II (very good-good). However, very rarely cases of water metamorphose are met. Thus, if for many years the Prut waters, according to the classification of Aleokin (1970) referred to the hydrogencarbonate class, group of calcium, type II (CCa II), than in March 2014 at Slobozia Mare station and in July 2014, downstream to Gotesti, the water changed to hydrogencarbonate class, group of sodium, type II (CNa II). This essentially changes the properties of waters for irrigation, despite the fact that the values of mineralization allow using water for irrigation purpose