For nanoporous silicon treated with nitric acid, a remarkable photoluminescence enhancement effect is observed. The enhancement is controlled by irradiation with visible light. Photoluminescence. infrared spectroscopy and electron spin resonance are used to compare tile material, modified by nitric acid with as-prepared porous silicon and oxidized by irradiation in air. Characteristic changes in the infrared spectrum during the enhancement indicate that an unidentified hydrogen source, activated by light, could terminate the surface dangling bond centres formed by the HNO3 oxidation process.