A method for simultaneous determination of potassium and caesium has been developed. The method involves quantitative precipitation of K2Pb[Cu(NO2)6] and Cs2Pb[Cu(NO2)6], and then thermal decomposition at 400-degrees-C and determination of the loss in mass or the amount of nitrogen oxides released. The results of the analysis are calculated on the basis of calibration curves for the loss in mass or conductivity, determined through the decomposition of standard samples.