Variations in Stoichiometry of SrTiO3 (001) surfaces with high-temperature exposure to reducing environments were quantified with Rutherford backscattering and Auger electron spectroscopies. In addition to the expected oxygen loss, cation-cation ratios varied systematically. Specifically, the surface becomes Ti- and O-deficient at all temperatures. The associated surface structures determined by scanning tunneling microscopy showed that the surface accommodates nonstoichiometry by adopting periodic structures of characteristic suboxides with the formula Srn+1TinO3n+1. The chemical and structural variations are discussed in terms of mass transport models for sublimation and segregation. Since previous surface studies had been concerned with effects of sputtering or the behavior of stoichiometric surfaces, the results presented here represent the first systematic study of compositional variations upon annealing of the (001) surface.