Recent progress in the development of a β-alumina gas sensor for automotive applications is reported. The sensing device consists of two solid electrolytes (namely a thin film of sodium sulfate deposited on β-alumina by appropriate treatment in a gaseous atmosphere), with two electrodes having different catalytic properties, one made of platinum, the other of gold. The β-alumina component was prepared as sintered pellets by pressing and as thick films by screen-printing. The electrical response of these materials at different temperatures in the range 300° to 800°C and under various gases (CO, NO, NOx) with dilution in pure air in the range 5 ppm to 5 vol% was investigated and studied as a function of the preparation technique. The dense ceramic and the thick film designs behave similarly. A sensor prototype based on the thick film design was then developed and tested. All the experimental results prove that the sensor can be successfully used for selective detection of CO and NOx. The selectivity results from an appropriate choice of the working temperature of the sensing element. At low temperatures (300 to 400°C) NO2 can be selectively detected in the presence of CO; the opposite happens in the higher temperature range (550 to 650°C).