A new system based on a Co film covered by ZnO nanorods (NRs) has been fabricated with sputtering technique. Its stability and magneto-optical properties are suitable for magnetic gas sensing applications. The tests have been performed at room temperature exploiting a magneto-optical Kerr effect magnetometer using H2 and CO as target gases. The reaction process is affected by a thin interface constituted by Co-doped ZnO. The surface of ZnO NRs and the interface are the sensing elements of the sensor, while Co acts as a magnetic transducer. The sensor is sensitive, stable and fast as concern both the response and the recovery time. The used system for gas detection can be easily reduced in size allowing the development of a low-cost and portable sensor.