The effect of a reducing hydrogen ambient on textured tin oxide thin films on glass substrates has been investigated. Hydrogen treatments were done at 230 and 430 degrees C by hot wire (HW) and rf plasma-decomposed hydrogen with pure H-2 as source gas. By these treatments the possible reduction of the substrate during the deposition of a-Si:H for solar cells is simulated. Ion beam techniques revealed that the exposure to HW-decomposed H-radicals leads to the formation of a tin-rich surface layer of 40 nm in 1 min at both 230 and at 430 degrees C. The loss of oxygen is higher for the high-temperature treatment. The optical transmission at a wavelength of 800 nm is reduced from 80% to less than 20%, while the sheet resistance increases from 6 to 8 Ohm/square. At both temperatures the reduction of fluorine-doped tin oxide (FTO) by a HW-treatment occurs faster than by rf plasma-decomposed H. The H radical concentration, which is higher for the HW-decomposed hydrogen as compared to rf plasma-decomposed hydrogen, is the most important factor in determining the rate of the reduction process. For short exposures to H radicals, the transparency and conductivity of the tin oxide may be completely restored by means of reoxidation in air at 400 degrees C. In contrast, prolonged exposure to H-radicals induces irreversible loss of transparency and conductivity, concomitant with formation of granule-like particles of metallic tin on the surface. A thin plasma-deposited a-Si:H-layer was found to effectively protect the FTO-layer against reduction due to HW-generated H-radicals.