A comparative study was performed on two TiO2 (110) surfaces: one annealed in vacuum, the other annealed in hydrogen. Local geometric and electronic structures were characterized using scanning tunneling microscopy and spectroscopy, in conjunction with conventional surface analytical techniques. Each surface exhibited a distinct pattern of electronic and geometric inhomogeneity. The origin of the defect structure resulting from the two treatments is discussed in terms of reduction mechanisms, defect formation thermodynamics, and chemical reactivity. The implication of local variation in the defect structure to the TiO2 interaction with reactive species is also addressed.