The uptake of HNO3 on aviation kerosene (TC-1) soot was measured as a function of temperature (253-295 K) and the partial pressure of HNO3, and the uptake of HNO3 on hexane soot was studied at 295 K and over a limited partial pressure of HNO3. The HNO3 uptake was mostly reversible and did not release measurable amounts of gas-phase products such as HONO, NO3, NO2 or N2O5. The heat of adsorption of HNO3 on soot was dependent on the surface coverage. The isosteric heats of adsorption, Delta H-0(isosteric), were determined as a function of coverage. Delta H-0(isosteric) values were in the range -16 to -13 kcal mol(-1). The heats of adsorption decrease with increasing coverage. The adsorption data were fit to Freundlich and to Langmuir-Freundlich isotherms. The heterogeneity parameter values were close to 0.5, which suggested that a HNO3 molecule can occupy two sites on the surface with or without being dissociated and that the soot surface could be nonuniform. Surface FTIR studies on the interaction of soot with HNO3 did not reveal formation of any minor product such as organic nitrate or nitro compound on the soot surface. Using our measured coverage, we calculate that the partitioning of gas-phase nitric acid to black carbon aerosol is not a significant loss process of HNO3 in the atmosphere.