SnOz-based gas sensors containing 0.05, 0.1, 1 and 3 mol% Pd are constructed using thick-film technology. SnOz powder is prepared by two processes in which Pd is added to the base material before calcination either as the hydroxide or chloride. Sensor sensitivity to CO is studied in a heated test chamber at temperatures ranging from 300 to 600 "C. NO and SO;!, which exist in combustion gases, are used as interference gases at concentrations of 50 and 200 ppm, The carrier gas is a dry synthetic combustion gas consisting of 80% N2, 15% CO* and 5% OZ. The CO response of the sensors decreases markedly when NO is introduced into the test chamber. The effect is strongest at low test temperatures and with sensors in which the concentration of Pd is high. It is practically negligible at 550 "C. Interference by SO* also appears to be temperature dependent, causing an increase in the conductance response to CO at temperatures of up to 450 "C. When the temperature is raised to 500 "C, and further to 600 "C, SO2 appears to cause a decrease in the CO response. The CO response of sensors in which Pd has been added in chloride form is higher, especially at temperatures between 550 and 600 "C. The best sensitivity is achieved with sensors containing 0.05 mol% Pd. Sensors containing 0.05 mol% Pd and operated at about 500 "C could serve as indicators of CO concentration in combustion gases. The thick-film paste containing Pd added to SnOz in the chloride form yields good response characteristics.