The results presented in this paper are obtained using (\mathrm{SnO}{2})-based gas sensors as chromatographic detectors. The tests were performed during a temperature cycle consisting in steps of one week at (350,400,450,400) and (350^{\circ} \mathrm{C}). The analysed gas samples were (\mathrm{CO})-air mixtures with (\mathrm{CO}) concentration between 0 and (50 \mathrm{ppm}). The advantages of the used experimental set-up are the stable ambient atmospheric conditions (the constant flow of the stable carrier gas) and the separation properties of the chromatographic column which compensate the intrinsic lack of selectivity of (\mathrm{SnO}{2})-based gas sensing layers; the chromatographic column separates the gas mixture into a time sequence, so the sensor receives the different gases at different moments. The compositions of the used eluent gas (-\mathrm{O}{2}) technical grade which also contains percents of (\mathrm{N}{2}) and about 40 ppm of (\mathrm{CH}{4}) - and of the analysed sample can be examined in the same experimental run of the response to (\mathrm{O}{2}, \mathrm{CH}{4}) and (\mathrm{CO}) because, when gases are present in the sample which are also present in the carrier gas, differences appear between the concentrations in the gas sample and in the carrier gas in the ambient atmosphere of the sensor at their corresponding moments in the time sequence. It was found that the relative sensitivities to (\mathrm{O}{2}) and (\mathrm{CH}_{4}) increase when the temperature increases; the relative sensitivity to (\mathrm{CO}) decreases when the temperature increases and is strongly influenced by the temperature cycle. Its value at (350^{\circ} \mathrm{C}) and (40 \mathrm{ppm}) is around (40 %) before the cycle and around (16 %) after. It was also found that after the period of heating at (450{ }^{\circ} \mathrm{C}), the conductance at 400 and (350^{\circ} \mathrm{C}) became an increasing function of time; for a week this process did not stop. Using the results listed above, an explanation is given: at (450^{\circ} \mathrm{C}) the characteristics of the surface of the sensing layer are modified by a new equilibrium of surface oxygen vacancies.