The principle of the method specified is adding a caesium chloride solution to the sample as an ionization suppressant, aspirating the sample directly into the air/acetylene flame of an atomic absorption spectrometer and measuring the absorbance at a wavelength of 589,0 nm. Applies to water samples with a mass concentration of potassium in the range from 5 mg/l to 50 mg/l. This range can be extended if other dilution factors are chosen.