The results of an investigation of the mercury‐mercury (II) halide electrode in liquid ammonia are reported. Variation of electrode potential with mercury(II) ion concentration was determined by measurement in concentration cells. These data have been interpreted and the activity coefficients of mercury(II) iodide, alone and in potassium iodide solutions, are estimated. Harned's rule was found to be obeyed in the mixed‐electrolyte solutions.It is shown that the mercury‐saturated mercury(II) chloride electrode is a reversible electrode that rapidly establishes and maintains the same potential within limits which permit its use as a working reference electrode in liquid ammonia. The potential of this electrode with respect to the standard mercury‐mercury(II) iodide electrode was found to be .A direct derivation of Harned's rule in functional form is illustrated and discussed. Variation of the coefficient in dilute solutions is verified by experimental results, which also suggest that these coefficients are independent of solvent and dielectric constant, and are a function of temperature at higher concentrations.