Nanosized tin dioxide samples (mean particle size: 3 nm, surface area: 170 m² g⁻¹) have been prepared and studied by UV–VIS–NIR diffuse reflectance spectroscopy, transmission electron microscopy, EPR and electrical conductivity measurements. The absorption threshold due to the SnO₂ interband transition appears near 340 nm (3.6 eV) compared to about 1770 nm (0.7 eV) for orthorhombic SnO. When treated in reducing conditions (vacuum, hydrazine), SnO₂ shows also a broad absorption in the visible range (350–600 nm) which disappears upon heating in oxygen. This broad absorption is also recorded upon direct heating in oxygen. This spectral feature is ascribed to the formation of oxygen vacancies, which depends on both oxygen pressure and temperature. The variation of electrical conductivity with oxygen pressure P, expressed by σ = k P^−1/4, shows that singly ionized anionic vacancies are the main structure defects, in agreement with EPR measurements. An original method of preparation of colloidal SnO₂ is also presented; the samples obtained show a still larger surface area (250 m² g⁻¹), a very narrow particle size distribution (1.1–2.4 nm), a high resistance to sintering and a low propensity to nonstoichiometry.