Thin amorphous Si1-xBx films, with x0 ranging from 0 to 0.5, were coevaporated onto preoxidized (100) Si wafers and quartz substrates, by using a dual‐electron‐gun high‐vacuum system. In order to study how the structural and optical properties depended on concentration and annealing temperature, heat treatments of the films were carried out at temperatures from 400 up to 1000 °C. The films were characterized by means of transmission electron microscopy, Auger electron spectroscopy, and spectrophotometry. It is shown that: (i) An amorphous Si1-xBx alloy can exist up to very high temperatures (≥1000 °C), when x is larger than ∼0.4; (ii) at 400 °C the optical band gap of an amorphous Si1-xBx alloy has the maximum value (1.59 eV) for x0=0.02 and then decreases to lower values for higher concentrations (e.g., 1.18 eV for x0=0.3); (iii) the measured optical band gap of amorphous Si1-xBx increases gradually with increasing annealing temperature up to 700–900 °C, and then increases rapidly when annealed at a higher temperature by about 0.5 eV. These changes can be associated with microstructural alterations. The stable amorphous Si1-xBx alloy with x0=0.3 has, for instance, a measured band gap of 1.23 and 1.74 eV after annealing at 800 and 900 °C, respectively. The relationship between the microstructure and the band gap of the films is discussed.