Tetragonal SnO2 nanostructures with different sizes, band gaps, and defects were synthesized by varying the amounts of ammonia and polyethylene glycol (PEG) used during production and their structural and optical properties were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) crystallography, BET surface area analysis, thermogravimetric analysis, Raman spectroscopy, UV–visible absorption spectroscopy, photoluminescence imaging, and X-ray photoelectron spectroscopy. In addition, the CO oxidation activity was examined by temperature-programmed reduction and temperature-programmed CO oxidation measurements. SEM and XRD analysis revealed that the particle size decreased with increasing PEG, but increased with increasing ammonia. Additionally, the band gaps decreased with increasing ammonia, but not with increasing PEG. Tetragonal SnO formed when larger amounts of ammonia were used, and this was converted to SnO2 upon annealing at temperatures up to 700°C. The SnO2 showed a unique strong green emission at 560 nm, which was attributed to a new oxygen deficiency. In addition, a sharp (328 nm) and two broad (390 and 460 nm) photoluminescence peaks corresponding to gap emission and the oxygen vacancies, respectively, were observed. The difference in CO oxidation activity with SnO2 was attributed to varying sizes and defects formed in response to preparation under different reaction conditions.