In this article, the adsorption behavior and the surface redox reactivity among small molecules including N2O, O-2, H2O and HCOOH on TiO2(110) surfaces are compared. New measurements of the interaction of HCOOH with defect-free and defective TiO2(110) surfaces and initial results from N2O adsorption on defects are combined with earlier work involving O-2 and H2O. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) were used to monitor the amount of species adsorbed on the surface and the presence of electronic defects. The defective surfaces were prepared by Ar+ bombardment and electron-beam exposure. Both O-2 and N2O "heal" defects (defect healing means an observation of the reducing of defect state intensity in both XPS and UPS spectra) even if they do not readily stick on the TiO2(110) surfaces at 300 K. H2O also heals defects, but at a much slower rate than either O-2 or N2O. In contrast, HCOOH exposure produces little healing even though the rate of adsorption for HCOOH (with saturation exposure of 10(3) L) is much faster than for H2O. The interactions of small molecules with defective TiO2(110) surfaces involve not only the adsorption and dissociation of molecules on the surface but also the surface redox reaction in which the reduced Ti states (such as Ti3+) are oxidized by the surface adsorbates to Ti4+, resulting in the reduced Ti 3d defect intensity.