The diffusion mechanism of tin into glass was investigated using a lab-scale float apparatus, in order to determine the reasons for the characteristic tin penetration profile of float glass. Tin penetration profiles of glass samples heated at various temperatures, times and atmospheres were measured by means of SIMS. The tin enriched inner layer, that is characteristic of the tin penetration profile of float glass, was seen to be formed by heating at more than 800^oC. It was found that the depth of the tin enriched inner layer was proportional to the holding time at the maximum temperature during the heat treatment, and was inversely proportional to the Fe³⁺ concentration in the glass. It was also proven that the tin enriched inner layer was formed by penetration of hydrogen from the atmosphere through the molten tin into the glass. These facts indicate that the reaction between hydrogen and Fe³⁺ is involved in the formation of the tin enriched inner layer. Consequently, it has been proposed that the mechanism of formation of the tin enriched inner layer is governed by two redox reactions and the diffusion behavior of both Sn²⁺ and Sn⁴⁺. Namely, one of these two redox reactions is the reduction of Fe³⁺ to Fe²⁺ due to hydrogen, resulting in the formation of a reduced surface layer. Another is the oxidation of Sn²⁺ to Sn⁴⁺ due to Fe³⁺ in the glass. These analytical results leading to a successful control of tin penetration into glass during the float process are discussed in detail.