The interaction of acetic acid with thin ammonium nitrate (AN) films has been studied using a Knudsen cell-flow reactor coupled with Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS). The gas phase was monitored with a quadrupole mass spectrometer (MS). The combination of mass spectrometry and FTIR-RAS allows for the simultaneous observation of the gas and condensed phases, respectively. Initial uptake coefficients (gamma values) and coverages (theta values) were determined from MS data over the temperature range of 200-240 K. The initial uptake coefficient varied from gamma = 0.058 to 0.0024 for temperatures from 200 to 240 K, respectively, and was independent of pressure over the range 5.7 x 10(-8) to 5.1 x 10(-6) Torr. The temperature-dependent uptake coefficients were analyzed using a precursor-mediated adsorption model to determine DeltaH(obs) = -32.3 kJ mol(-1) and DeltaS(obs) = -182.2 J K-1 mol(-1). In the absence of water vapor [relative humidity (RH) < 1%], integrated coverages for acetic acid on ammonium nitrate ranged from theta = 19.6 monolayers (MLs) (9.4 x 10(15) molecules cm(-2)) at 200 K to theta = 0.40 ML (2.1 x 10(14) molecules cm(-2)) at 240 K. The uptake was largely irreversible, with similar to15% of the adsorbed molecules isothermally desorbing from the film. The IR spectra revealed that acetic acid ionized on the surface, despite the fact that the ammonium nitrate film was effloresced. Adding small amounts of water vapor (4% RH) to the chamber resulted in unsaturated uptake and dramatically increased values of theta and gamma. Higher RH resulted in further increases of both theta and gamma. The IR spectra again revealed that acetic acid ionized on the surface. Furthermore, the formation of a liquid layer was observed in the infrared when the acetic acid was taken up by the film at RH > 10%. This observation suggests that the adsorption of water-soluble organics onto inorganic aerosol may dramatically enhance the ability of the aerosol to take up water at low RH.