Measurement of Na, K, Mg and Ca concentrations in natural fresh (non-mineralized) water samples can be performed in the same sample solution with little or no dilution, in acetylene-air flame with the addition of lanthanum chloride (La 15 g/L), caesium chloride (Cs 1.0 g/L) and hydrochloric acid (HCl 2.0% V/V) by flame atomic emission (Na, K) and absorption (Mg, Ca) spectrometry. Examined remaining influence of other major elements and main interfering components (i.e. sulphates(VI) and silicon) on measured elements was found to be minimal. The recoveries for various concentrations of individual elements in test solutions that had considerably different concentrations of Na, K, Mg and Ca and various participation of sulphates and silicon as well as different values of mineralization were found to be 98.9÷100.6 %, 99.2÷100.6 %, 99.1÷100.9 % and 99.1÷100.9 % for sodium, potassium, magnesium and calcium, respectively.