Amorphous tin oxide films, 100–800 nm thick and of resistivity ~6–8 mΩ cm, were deposited on glass substrates using a filtered vacuum arc with an oxygen background gas pressure of 4.0 mTorr. The films were annealed in air at a temperature of 300°C for 1, 3, 5, 7, and 10 min. Film morphology, structure, composition, roughness, and light transmission were determined before and after the annealing, on cold samples, with atomic force microscopy, x-ray diffraction diagnostics, x-ray photoelectron spectroscopy, and light transmission meter. The roughness depended weakly on the annealing time, and decreased with the thickness of the film. The film transmission in the visible region was practically independent of the annealing time. Film conductivity increased with the annealing time, reaching a maximum value after 3–7 min, larger by a factor of 2.0–2.9 than that measured before annealing. The oxygen to tin density ratio on the film surface decreased relative to its value before annealing and reached a minimum after annealing for 7 min. After annealing for 10 min, the O/Sn ratio increased relative to the minimum value but was lower than the ratio before annealing. The O/Sn ratio in the bulk decreased monotonically for annealing times longer than 1 min. The film conductivity before and after annealing depended linearly on the film thickness. A model is proposed to elucidate the dependence of the conductivity on the annealing time and on the film thickness.