The activity of the hydrogen ion (H+) is estimated in sulfuric acid aqueous solutions of concentrations 0.25 to 2.0 mol.L(-1) at temperatures 298 to 373 K (25 to 100 degrees C). This is done by following an existing "extended Debye-Huckel" equation, which was originally developed only for the case of pure H2SO4 solutions at ambient temperature. The same equation is applied for the evaluation of the activity of H+ in mixed metal sulfate solutions, particularly for the aqueous system H2SO4-ZnSO4-FeSo(4)-Fe-2 (SO4)(3). The estimation of the activity of H+ by this method requires the knowledge of the molality of free H+ ions and the true ionic strength (in molal scale). Consequently, an algorithm is advanced for the calculation of species concentrations in such mixed solutions. Estimated pH values agree very well with experimentally measured ones.