The interaction of acetaldehyde with TiO(2) nanorods has been studied under low pressures (acetaldehyde partial pressure range 10(-4)-10(-8) Torr) using chemical ionization mass spectrometry (CIMS). We quantitatively separate irreversible adsorption, reversible adsorption, and an uptake of acetaldehyde assigned to a thermally activated surface reaction. We find that, at room temperature and 1.2 Torr total pressure, 2.1 +/- 0.4 molecules/nm(2) adsorb irreversibly, but this value exhibits a sharp decrease as the analyte partial pressure is lowered below 4 x 10(-4) Torr, regardless of exposure time. The number of reversible binding sites at saturation amounts to 0.09 +/- 0.02 molecules/nm2 with a free energy of adsorption of 43.8 +/- 0.2 kJ/mol. We complement our measurements with FTIR spectroscopy and identify the thermal dark reaction as a combination of an aldol condensation and an oxidative adsorption that converts acetaldehyde to acetate or formate and CO, at a measured combined initial rate of 7 +/- 1 x 10(-4) molecules/nm(2) s. By characterizing binding to different types of sites under dark conditions in the absence of oxygen and gas phase water, we set the stage to analyze site-specific photoefficiencies involved in the light-assisted mineralization of acetaldehyde to CO(2).