Here, the splitting of water from humidity at the surface of WO 3 is reported. The production of hydrogen occurs under mild conditions, at 300 °C in the presence of humidity. Using operando diffuse reflectance infrared Fourier transform spectroscopy and DC resistance measurements complemented with DFT calculations, it was possible to elucidate the temperature dependent surface reactions of WO 3 with humidity. By examining multiple different nanomaterials at 300°C, it was found that the oxidation of the surface by humidity is generally valid for WO 3. The evolution of hydrogen was detected using catalytic conversion measurements. In addition to the evolution of hydrogen, understanding the interaction between the surface and humidity is a prerequisite for the optimized use of WO 3 for a wide array of applications, such as optical sensors or memristors.