A Pitzer ion interaction model has been applied to the systems H 2SO4?H2O (0?3 mol kg?1, 0?55 °C) and H2SO4?(NH4)2SO 4?H2O (0?6 mol kg?1, 25 °C) for the calculation of apparent molar volume and density. The dissociation reaction HSO4?(aq) ? H+(aq) + SO42?(aq) is treated explicitly. Apparent molar volumes of the SO42? ion at infinite dilution were obtained from part 1 of this work,(1)and the value for the bisulfate ion was determined in this study from 0 to 55 °C. In dilute solutions of both systems, the change in the degree of dissociation of the HSO4? ion with concentration results in much larger variations of the apparent molar volumes of the solutes than for conventional strong (fully dissociated) electrolytes. Densities and apparent molar volumes are tabulated. Apparent molar volumes calculated using the model are combined with other data for the solutes NH4HSO4 and (NH4) 3H(SO4)2 at 25 °C to obtain apparent molar volumes and densities over the entire concentration range (including solutions supersaturated with respect to the salts).