The calculation of thermodynamic properties of many strong electrolytes in solution, including aqueous sulfuric acid, has been performed over the past four decades using so-called thermodynamic models, such as the well-known Pitzer model. I have recently pointed out (Fraenkel, 2012) [15,16] that H2SO4 in water appears to follow the mean ionic activity pattern of a strong 1-3 electrolyte, and postulated that this H(3)A acid may be H4SO5 fully ionizing to 3H(+) (3H(3)O(+)) and HSO53-. This contrasts with the traditional view of the aqueous acid - claimed to be supported by thermodynamic models - according to which H2SO4 retains its molecular structure in water and dissociates primarily to H+ and HSO4-<-,and at <0.1 M, HSO(4)(-<-)dissociates further to H+ and SO42-. I now show that a good fit of Pitzer model with the activity coefficients reported by Hamer and Harned can be obtained for the "1-3 H2SO4'' even by using the simple 3-parameter equation of the model; the best-fit Pitzer parameters are beta((0)) = 0.240, beta((1)) = 4.30 and C-MX = -0.0134, and the standard deviation, sigma is 0.0152. With the corrected activity coefficients as proposed in the first reference above, the best-fit parameters are beta((0)) = 0.230, beta((1)) = 3.60 and C-MX = -0.0120, and sigma = 0.0081. sigma of the analysis of the "1-3 acid'' is in both cases considerably lower than that of the "1-2 acid'' (sigma = 0.049) that provides a best-fit beta((1)) value of -3.000; a negative beta((1)) is inappropriate since it is parallel to a negative ion-ion distance of closest approach in Debye-Huckel-type expressions of the activity coefficient. (C) 2014 Elsevier Ltd. All rights reserved.