The uptake of peroxyacetyl nitrate (PAN) on liquid sulfuric acid surfaces has been investigated using a fast-flow reactor coupled to a chemical ionization mass spectrometer. PAN was observed to be reversibly adsorbed on sulfuric acid. From the time-dependent uptake, Henry's law solubility constant of PAN in sulfuric acid was obtained. The measured solubility constant was found to depend strongly on temperature (199--226 K), but weakly on acid composition (46--72 wt %). On 72 wt % H2SO4, for example, the effective Henry's law solubility constant was determined to be about 5.6¿104 and 3.1¿103M atm-1 at 208 and 222 K, respectively. Heterogeneous reactions of PAN with HCl, Cl, ClO, and OClO on liquid sulfuric acid were also investigated and found to be very slow (&ggr;<1¿10-4). Atmospheric implications of the present results are discussed.¿ 1997 American Geophysical Union