The thermal oxidation rate of Si in dry oxygen is increased by illumination with low-power visible and ultraviolet laser light and is found to be a function of the wavelength of the laser radiation. Visible laser radiation increases the thermal oxidation rate of Si by approximately 40%, and ultraviolet radiation at 350 nm by 60%, over the temperature range of 770–900 °C. A laser- induced rate increase of 40% is also observed in wet (H2O+O2) thermal oxidation. However, no significant wavelength dependence is seen in this case. It may be concluded from these data that ultraviolet laser light affects some component of the dry oxygen reaction that is not present (or not important) in wet thermal oxidation. There appears to be a threshold energy of 3.0 to 3.5 eV for this observed optical effect. Possible mechanisms for the optical interaction are discussed.