Context <p>Chlorine properties that affect its reaction with the SnO₂ sensor surface are discussed. This includes temperature variation and Cl₂ reaction with humidity. Transition state theory formalism evaluates related thermodynamic properties such as Gibbs free energy and its components, enthalpy, and entropy. Logistic functions determine the effective concentration of Cl₂ gas due to its reaction with humidity and sensor material. The Gibbs free energy of adsorption and transition or activation is evaluated as a function of temperature. Results include SnO₂ sensor response to Cl₂ gas as a function of temperature and Cl₂ concentration. Results also include response time and the effect of humidity. An optimum response temperature can be between room temperature and 200 °C. A comparison with available experimental results is performed, which shows a good agreement between theory and experiment. The present model is the only available model that can successfully compare the theory and experiment of response and response time, including temperature and humidity effects.</p> Methods <p>Gaussian 09 software package is used with B3LYP level of DFT since most previous successful gas sensor calculations are performed using this version of DFT. 6-311G** basis sets are used to represent oxygen and chlorine atoms, while SDD functionals are used to represent heavier Sn atoms.</p>