AbstractE. m. f.'s of the cell, H2(g,p atm)/HClO4(m1), NaClO4(m2)/HClO4(m1), Hg2(ClO4)2(m3)/Hg were measured at 20°, 25°, 30° and 35°C. The E° value is expressed as E° = 0.8915–5.14×10−4T+5.70×16−7T2. The free energy for the reaction, Hg2(aq)+H2(g)=2Hg(l)+2H+(aq) is given by ΔF°= −41,119+23.70T‐26.3×10−2T2. The thermodynamic quantities, ΔF° = −36,389, ΔH° = −38,782 cal mole−1and ΔS° = −8.03 cal deg−1mole−1are given for the reaction, and ΔFlo=36.39, ΔHfo=39.78 Kcal mole−1and S° = 13.82 cal deg−1mole