Isopiestic molalities of LiHSO4(aq), KHSO4(aq), and CsHSO4(aq) have been measured over the temperature range 383.46 K to 498.19 K with NaCl(aq) serving as the reference electrolyte for the calculation of excess thermodynamic quantities. Molalities of the reference electrolyte ranged from about 0.5 mol . kg(-1) to greater than 6 mol . kg(-1). The ion-interaction model of Fitter gave an excellent description of the experimental results when the solutions were treated as mixed-electrolyte systems resulting from partial ionization of HSO4-(aq). A simple polynomial model for the temperature dependence of the ion-interaction parameters is sufficient to reproduce the experimental results. When calculated as stoichiometric activity coefficients for 1-2 charge types, the resulting quantities are quite small, especially when compared with activity coefficients based on the assumption of a 1-1 charge type. The activity coefficients are consistent with limited dissociation of the hydrogensulfate anion which decreases with increasing temperature.