A method of determination of high-frequency conductivity in aqueous electrolyte solutions is suggested. It is based on measurement of the rate of simultaneous heating of two liquids in a capacitor gap by high-frequency electric field. The equality of electric field strengths was provided by placing the cuvettes with the solutions in parallel to each other and joining them together by two common electrodes. The ratio of the heating rates of the electrolyte and water measured under this condition allows determining the ratio of their conductivities. The device used for high-frequency heating was an ultrahigh-frequency treatment device that allowed exposing the objects both to electric and magnetic field with the frequency of 27.12 MHz. The regularities of experimental determination of the heating rate of the studied liquids by electric and magnetic fields are provided. The rates of heating water and NaCl and KCl solutions in the concentration range of 0.004–20% by high-frequency electric field are determined. It is found that the solution heating rate exceeds the heating rate of distilled water by many times. The maximum heating rate and limiting high-frequency conductivity corresponding to the concentration of 0.5% for the both solutions exceed these parameters by approximately 60 times. Direct proportionality between the static conductivity of the electrolytes and the rate of their heating by high-frequency magnetic field is confirmed in the concentration range of 1–20%.