The interaction of HO2NO2 (peroxynitric acid, PNA) vapor with liquid sulfuric acid was investigated for acid contents ranging from 53 to 74 wt % and at temperatures between 205 and 230 K, using a fast-flow reactor coupled to a chemical ionization mass spectrometer. PNA was observed to be physically taken up by liquid sulfuric acid, without undergoing irreversible aqueous-phase reactions. From the time-dependent uptake, the quantity H*root D-1 (that is, the product of the effective Henry’s law solubility constant and the square root of the liquid-phase diffusion coefficient) was obtained. The effective Henry’s law solubility constant H* of PNA in liquid sulfuric acid was derived by estimating the liquid-phase diffusion coefficient based on a cubic cell model. In general, the solubility was found to increase with decreasing acid content and decreasing temperature. The heterogeneous reaction between PNA and HCl in liquid sulfuric acid was also examined and was found to be very slow (gamma < 1 x 10(-4)). The measured solubility reveals that PNA should exist predominately in the gas phase under conditions characteristic of the middle or lower latitude stratosphere. For winter polar stratospheric conditions, however, incorporation of PNA into sulfate aerosols may lead to redistribution of PNA from the gas to condensed phases, potentially affecting stratospheric HOx and NOx concentrations.