SBA-15 (2D hexagonal structure) and KIT-6 (3D cubic structure) silica materials are used as templates for the synthesis of two different crystalline mesoporous WO3 replicas usable as NO2 gas sensors. High-resolution transmission electron microscopy (HRTEM) studies reveal that single-crystal hexagonal rings set up the atomic morphology of the WO3 KIT-6 replica, whereas the SBA-15 replica is composed of randomly oriented nanoparticles. A model capable of explaining the KIT-6 replica mesostructure is described. A small amount of chromium is added to the WO3 matrix in order to enhance sensor response. It is demonstrated that chromium does not form clusters, but well-distributed centers. Pure WO3 KIT-6 replica displays a higher response rate as well as a lower response time to NO2 gas than the SBA-15 replica. This behavior is explained by taking into account that the KIT-6 replica has a higher surface area as demonstrated by Brunauer–Emmett–Teller analyses and its mesostructure is fully maintained after the screen-printing step involved in sensors preparation. The presence of chromium in the material results in a shorter response time and improved sensor response to the lowest NO2 concentrations tested. Electrical differences related to mesostructure are reduced as a result of additive introduction.