Abstract: A method of producing a mixed electrolyte powder having an electrolyte ion composition suited for use in preparing a dialyzing fluid for perfusing artificial kidney systems for bicarbonate dialysis, which method comprises a step of pulverizing the whole or part of sodium chloride, which is one of the components of the above powder, in a micronizer to a microfine powder having a particle size of about 20-30 .mu.m and adding thereto 2-4 percent by weight of glacial acetic acid by spraying, whereby a microfine powder of sodium chloride acidified with acetic acid is obtained, a step of dissolving other components constituting the above mixed powder and, if any, the remaining portion of sodium chloride in warm water and spray-drying the solution using a spray nozzle or disk atomizer to a microfine powder having a particle size of about 20-100 .mu.m, and a step of mixing both the microfine powders obtained in the above steps together.