Sm³⁺-doped glass sol was prepared by a sol–gel method and coated on a bare Si substrate and an Al coated Si substrate (Al/Si). The coated glass films were heat treated in a hydrogen atmosphere or air to reduce the Sm³⁺ to Sm²⁺ and then the optical properties were examined through a photoluminescence (PL) experiment. While the glass films coated on a bare Si substrate or an Al/Si one were well reduced in hydrogen atmosphere, only the glass films coated on the Al/Si substrate were reduced in air. We thus suggest two possible reducing mechanisms related to hydrogen and the metal aluminium and we found that the metal aluminium coated on a Si substrate plays an important role in the reducing process.