Abstract: TiO2 -based sensors were developed and characterized for improvement of selectivity and sensitivity towards CO gas at high temperatures. Selectivity was improved by combining several sensors into an array, while sensitivity improvement was studied by examination of the effects of various metal and metal oxide additives and reactive sputtering film preparation. TiO2-based sensors containing La2O3 and La2O3/CuO were combined into an array for testing of CO and O2 gas mixtures. Nonlinear regression analysis was developed to determine the concentrations of gases in the mixture. The regression technique can be used to determine the prediction ability of gas sensor combinations for given gas concentration ranges using a defined orthogonality index, as well as to determine the amount of each gas in the mixture based on the responses of a set of sensors. The prediction ability for a La2O3-containing sensor combined with La2O3/CuO-containing sensors with varying levels of CuO are compared for a range of CO and O2 concentrations. Concentrations of CO and O2 in gas mixtures are predicted using the La2O3- and La2O3/CuO-containing sensors and the regression analysis. Addition of various metals and metal oxides, including Au, CuO and La2O3 to TiO2-based gas sensors has been examined to determine the effect of additives on the gas-surface reactivity and electrical response of the sensors. Electrical measurements show that sensor sensitivity is decreased upon addition of La2O3 with only a slight increase in sensitivity when CuO is added to La2O3-containing sensors. Au, however, was found to increase the sensitivity of the TiO2-based sensors with and without La2O3. Gas chromatography was used to determine the catalytic conversion of CO for the various sensor materials at different temperatures for comparison with electrical measurements. General guidelines for additive selection have been proposed based on the results. Reactively sputtered thin film TiO2 gas sensors have been fabricated and characterized for improved sensitivity to CO gas. Sensitivity of optimized thin film sensors to CO was found to be much higher than for traditional thick film TiO2-based sensors. Characterization of the phase and morphology of the sensors using XRD, TEM and AFM show that the film consists of a dense layer of TiO2 in the rutile phase, with surface roughness changing with film thickness. Sensitivity of the thin film sensor could be varied by changing the film thickness, with a film thickness of 240 nm showing the best sensitivity to CO gas, although response and recovery times were slow.