With silicon monoxide films thermally evaporated at various angles between the substrate normal and the vapor stream, we obtain a refractive-index change ranging from 2.04 to 1.27, with evaporation angles ranging from 0 to 80 deg, respectively. This method provides an alternative to evaporating with a partial pressure of oxygen, which is now used to control the refractive index. Also, the selectivity of the lower range of refractive-index values may satisfy the needs of special applications that require a value lower than what is available from common bulk materials.