The absorption edges of vacuum-evaporated silicon monoxide films are studied and a general equation based on the absorption being due to non-direct electronic transitions in k-space is found to be compatible with the experimental results. The optical energy gap, E opt, is determined from the high absorption region of the fundamental edge. Measurements on films deposited at different rates and having different thicknesses are analysed and discussed in terms of the values of E opt. The form of the absorption edge remains similar for annealed films but annealing has the effect of increasing the values of E opt. The experimental results are related to earlier published work based on measurements of electron spin resonance and refractive index of films prepared under different conditions and additional experiments are reported.