We demonstrate that one can detect minuscule amounts of hydrogen diffusion out of a-Si:H under illumination at room temperature by monitoring the changes in the Raman spectrum of a-WO3 as a function of illumination. The Staebler-Wronski effect, the light-induced creation of metastable defects in hydrogenated amorphous silicon (a-Si:H), has been one of the major problems that has limited the performance of solar cells based on this material. The recently suggested 'hydrogen collision model' can explain many aspects of the Staebler-Wronski effect, but assumes that the photogenerated mobile hydrogen atoms can move a long distance at room temperature. However, light-induced hydrogen motion in a-Si:H has not been experimentally observed at room temperature. We utilized the high sensitivity of the Raman spectrum of electrochromic a-WO3 to hydrogen insertion to probe the long-range motion of hydrogen at room temperature. We deposited a thin (200 nm) layer of a-WO3 on top of a-Si:H, and under illumination a change in the Raman spectrum was detected. By comparing the Raman signal changes with those for control experiments where hydrogen is electrochemically inserted into a-WO3, we can estimate semi-quantitatively the amount of hydrogen that diffuses out of the a-Si:H layer. Published by Elsevier Science Ltd.