Equilibrium water partial pressures (pH(2)O) above 3.69(5), 5.67(2), and 8.65(1) mol kg(-1) NH4HSO4(aq), and 10.19 mol kg(-1) H2SO4(aq), have been determined using a capacitance manometer over the temperature ranges 258.1(1)-239.5(6) K and 298.1(2)-238.5(2) K, respectively. Measured pH(2)O above the aqueous H2SO4 solution agree with values calculated using the model of Clegg et al. (J. Phys. Chem. A 1998, 102, 2137-2154) to within +5.3% to -2.8% at all temperatures. Results for the NH4HSO4(aq) solutions are also consistent with calculated pH(2)O, with deviations of +0.45% to +6.27% (measured -calculated) for all data. The formation of solids was observed in the NH4HSO4(aq) solutions at the following temperatures: 258.0(6) +/- 0.1 K [3.69(5) mol kg(-1)], 246.4(7) +/- 0.1 K [5.67(2) mol kg(-1)], and 261.0(1) +/- 0.1 K [8.65(1) mol kg(-1)]. The data are consistent with predictions of ice precipitation in the 3.69(5) mol kg(-1) test solution, letovicite ((NH4)(3)H(SO4)(2(cr))) in the 8.65(1) mol kg(-1) solution, and both ice and letovicite in the 5.67(2) mol kg(-1) solution. The liquid-phase compositions of the solutions containing precipitates are calculated for different temperatures. The results of the experiments are compared with solubility data from other studies and with model predictions of solid/liquid equilibrium for NH4HSO4-H2SO4-H2O mixtures to low temperature over a wide range of composition.