This invention relates to the field of nonferrous metallurgy, and in particular, to producing aluminum by the electrolysis of a melt and can be used in the process control of an electrolyte composition by quantitative X-ray phase analysis (XRD) of potassium-containing electrolyte with calcium or calcium and magnesium additives. The method is as follows. A quantitative XRD method is employed for analyzing doped samples of crystallized bath samples taken from baths. For this purpose, a weighted ground bath sample is mixed with a weighted quantity of sodium fluoride at a ratio, for example, 1:2 by weight of the sample. The weighted quantities are mixed and placed in a furnace heated to 650-750 degrees centigrade for 20-40 minutes to dissolve sodium fluoride in the sample and recrystallize the sample with the desired phase composition. Then the doped sample is placed in a furnace heated to a temperature of 420-450 degrees centigrade and held there for 15-30 minutes. The doped sample is then removed from the furnace and allowed to air cool. The phase composition of the doped sample is analyzed by any quantitative X-ray phase method. Based on quantitative data of the phase composition, NaF, KF, A1F3, CaF2, MgF2 concentrations in the doped sample are calculated. Taking into account the quantity of the introduced additional NaF, the composition and the CR of the initial sample are calculated. Doping the samples and the subsequent thermal treatment provides samples with equilibrium phase composition and with known crystalline phases, and good crystallinity of phases in the sample which is essential when using methods of quantitative XRD.