The properties of reactively sputtered ZnO thin films used as chemical sensors are investigated in N2, synthetic air, 10 ppm of NO2, 30 ppm of CO and in the presence of humidity. The behaviour is correlated to a grain boundary determined conduction model. Using the resistance ratio of twin sensors, one of which is coated with a thin Au catalytic layer, allows the sensor response to CO to be increased and the output signal to be stabilized against sensor drift; the response time is also reduced.