Designing a high temperature resistant IR-gas sensor for CO2 and water vapor one has to consider the differences in the specific absorbances, spacings between the heated parts and optics/electronics, emission spectrum of cost-saving radiators and high humidity in the measuring gas. Using HITRAN database a sensor model was developed. Water vapor can be measured selectively at 1.85 μm with a chamber length of 20 cm. So CO2 has to be measured at low wavelengths too. In the range of 2.7 μm CO2 absorbs sufficiently but a water absorption is to be considered. The model shows that the CO2 absorption reaches a maximum in the range from 2.7 μm to 2.75 μm with a decreasing water absorption with increasing wavelengths. After finishing the sensor the model was checked with the actual filter curves. The results show a good analogy between model data and measurements and the benefit of modeling in the design of IR-gas sensors. The analyze of cross sensitivity shows strong overlapping bands and a multiplicative influence of water absorption to the CO2 signal. After correction of the absorption signal of 2.7 μm-channel with the absorption signal of the 1.85 μm-channel the cross sensitivity to water vapor was limited to <3% of CO2 measuring range.