Semiconducting metal oxide-based gas sensors are an attractive option for a wide array of applications. In particular, sensors based on WO3 are promising for applications varying from indoor air quality to breath analysis. There is a great breadth of literature which examines how the sensing characteristics of WO3 can be tuned via changes in, for example, morphology or surface additives. Because of variations in measurement conditions, however, it is difficult to identify inherent qualities of WO3 from these reports. Here, the sensing behavior of five different WO3 samples is examined. The samples show good complementarity to SnO2 (the most commonly used material)-based sensors. A surprising homogeneity, despite variation in morphology and preparation method, is found. Using operando diffuse reflectance infrared Fourier transform spectroscopy, it is found that the oxygen vacancies are the dominant …