In 1935 Hamer (J. Am. Chem. Sec. 1935, 57, 9) and Harned and Hamer (J. Am. Chem. Sec. 1935, 57, 27) published the results of extensive e.m.f. measurements for the activities of H2SO4 (ag) from low to high molalities over the temperature range 273.15 K to 333.15 K, using the electrochemical cells : PtH-2(g, p = 0.101325 MPa)H2SO4(aq)PbO2(s)PbSO4(s)Pt, and PtH-2(g, p = 0.101325 MPa)H2SO4(aq)Hg2SO4(s)Hg(1)Pt. The e.m.f.s were reported only as least-squares functions of temperature at rounded molalities. These results were criticized severely in the ensuing years as yielding inaccurate values of water activities and mean activity coefficients at molalities above 3 mol.kg(-1), values of the standard potentials and standard enthalpies of the cell reactions that depend on the molality of H2SO4 (aq), and incorrect entropy changes for the cell reactions. However, there are more recent claims that these e.m.f.s are accurate. The original measured potentials for the first cell from (0.0004985 to 7.093) mol.kg(-1) at the temperature 273.15 K were located (Harned, H. S.; Hamer, W. J. U.S. Naval Research Laboratory Report No. P-1071, 1934), and the available evidence was critically examined. We conclude that their experimental e.m.f.s at this temperature are accurate to at worst (0.1 to 0.2) mV up to 2.2 mol.kg(-1) or higher, except for a large bias potential as is typical for this cell. At higher temperatures, the sums of the reported smoothed e.m.f.s for these two cells are consistent to a few hundredths of a mV up to (2 to 3) mol.kg(-1). For the individual cells in this molality region, however, the smoothed e.m.f.s yield standard potentials that show molality-dependent variations of(0.9 to 1.6) mV, with the size of this variation increasing with temperature. We attribute this variation partly to their graphical smoothing of e.m.f.s to round molalities. It is likely that the cells no longer behaved reversibly above (2 to 3) mol.kg(-1).