One mechanism of cirrus cloud formation is homogeneous ice nucleation within ammoniated sulfuric acid particles. An important constraint on ice formation is the (NH4)2SO4-H2SO4-H2O phase diagram. In the completed laboratory work, the NH4HSO4-H2O phase diagram is determined over the domain relevant to the troposphere. The H2O vapor pressures of aqueous NH4HSO4 solutions are measured from -30 to 25 ¿C, and the compositions of the aqueous phase in equilibrium with a solid phase (i.e., ice or letovicite) are determined at several temperatures. Below 25 ¿C and above 42 wt% NH4HSO4(aq), the first solid to form from supersaturated NH4HSO4(aq) is (NH4)3H(SO4)2(s), not NH4HSO4(s). Supercooling of nearly 20 K is required to initiate letovicite formation for 100 mL samples. The results of this study are in good agreement with the calculations of Clegg et al. [1998>. ¿ 1999 American Geophysical Union