Preventing a build- up of indoor pollutant concentrations has emerged as a major goal in environmental chemistry. Here, we have applied chemical ionization mass spectrometry to study the interaction of acetone, a common indoor air pollutant, with Degussa P25 TiO2, an inexpensive catalyst that is widely used for the degradation of volatile organic compounds into CO2 and water. To better understand the adsorption of acetone onto Degussa P25, the necessary first step for its degradation, the experiments were carried out at room temperature in the absence of UV light. This allowed for the deconvolution of the nonreactive and reactive thermal binding processes on Degussa P25 at acetone partial pressures (10(-7)-10(-4) Torr) commonly found in indoor environments. On average, 30% of the adsorbed acetone is bound irreversibly, resulting in a surface coverage of irreversibly bound acetone of similar to 1 x 10(12) molecules/cm(2) at 3-4 x 10(-5) Torr. Equilibrium and dynamic experiments yield a sticking coefficient of similar to 1 x 10(-4) that is independent of the acetone partial pressures examined here. Equilibrium binding constants and free energies of adsorption are reported.