Light-excitation of semiconducting metal-oxide (SMOX)-based gas sensors is a promising means to lower their operation temperature, thereby reducing power consumption, which would allow for their broader application. Despite increased research interest in light-excited gas sensors, progress has been slow because of a lack of mechanistic understanding. Notably, significant differences between light-excitation and, the better understood, thermal-excitation mechanisms have been identified. Insights from operando spectroscopic studies have been key to understanding the surface chemistry that determines the performance of thermally activated SMOX, but they have not yet been performed on illuminated sensors. Here, for the first time, we demonstrate that it is possible to perform operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements on sensors under illumination. We demonstrate the benefits of the approach and show that under light illumination the splitting of water on the WO3 surface is responsible for the increased resistance of the sensor during exposure to high humidity.