Plane-wave pseudopotential density functional theory calculations are used to investigate the details of acetaldehyde adsorption and thermal chemistry on the TiO2 (110) rutile surface. The adsorption properties of a single acetaldehyde molecule on models of stoichimetric, reduced, and oxidized TiO2 surfaces are first examined. The experimentally observed shift of the temperature-programmed desorption (TPD) maximum on increasing coverage is rationalized on the basis of the adsorbate to surface bonding mechanism. Examination of the thermal chemistry of acetaldehyde on models of the TiO2 (110) surface allows one to understand the diffusion of acetaldehyde molecules on the surface and the formation of butene, as is experimentally observed. This reaction takes place through a noncatalytic process where the bridge oxygen vacancies play a fundamental role. The molecular details of the process allow one to explain the low amount of butene found in the experiment. The study of the thermal chemistry of acetaldehyde...