Abstract: A sensor to detect gases and vapors, particularly carbon monoxide and water vapor, at relatively low concentrations includes a substrate having a layer on a face of the substrate. The layer comprises a semi conductor metallic oxide (such as stannic oxide), a catalyst (such as platinum black), and a rheological agent (such as kieselguhr or sepiolite). The rheological agent induces porosity into the surface of the layer. The rheological agent affects the mixing and processing of the layer, and aids binding, resulting in a sensor with greater sensitivity and faster response.