Abstract: A resistive gas sensor has a porous semiconducting oxide body in which the pore surfaces are decorated with a precious metal. The sensor operates at ambient temperatures for detection and measurement of target gases, especially carbon monoxide. The porous sensor body is formed of primary crystallites agglomerated together, the agglomerate size being less than 10 times the primary crystallite size, and the primary crystallite size having an average diameter of less than 5 micrometer. From 0.05 to 80% of the pore surfaces are covered by the metallic phase, the metallic phase consisting of particles having an average size of less than 50 nanometers. Presence of the target gas is indicated by a change in electrical resistance of the sensor.