The effect of annealing on the composition, crystal structure, surface features and electro-optical properties of tin mono-sulfide (SnS) films, deposited by thermal evaporation at 300 °C, has been studied. Elemental analysis of the films shows sulfur deficiency, which increases at higher annealing temperatures (Ta). The SnS structure in the as-deposited and annealed films remains orthorhombic. With an increase in Ta, the grain size and the surface roughness are reduced. The electrical resistivity also decreases with increasing Ta. The variation of activation energy and optical parameters with Ta has been explained by taking into account the degree of preferred orientation of the grains. The films annealed at 100 °C show some unusual features compared to those annealed at other temperatures.