Optical transmission spectra are investigated experimentally below the fundamental absorption edge in tin oxide thin film-based gas sensors. The spectra are studied under different conditions for gas chemisorption when the resistance response to gas is modified by heating of the sensors in various atmospheres or adding of impurity metals into tin oxide. Optical bandgap, refractive coefficient and optical absorption index are deduced from the experimental spectra at the conditions tested. The changes of the optical parameters are related with the electronic properties of the films that depend on surface defect states created by chemisorbed oxygen species. The resistance response to the gaseous surrounding is discussed on the basis of the analysis of the optical parameters. Some aspects of the mechanism of gas sensitivity are commented for tin oxide gas sensors.