The e.m.f. of the membrane cell H2(1 atm), HClO4(m1, fixed), ClO–4-responsive membrane, HClO4(m2, variable), H2(1 atm) has been measured in order to determine the mean molal activity coefficients for aqueous perchloric acid at 25°C. The liquid membrane used consists of a solution of tetraheptylammonium perchlorate in ethyl bromide, and is characterized by an ideally Nernstian response to the ClO–4 ions. The HClO4 concentration range covered is 0.0001 to 10 mol/kg, a range which extends well below the lower concentration limit typical of the isopiestic method (0.1 mol/kg).The present e.m.f. method is applicable to activity coefficient determinations for perchlorates other than HClO4.