The chlorine (Cl2) sensitivity of pristine and Pd-doped titanium dioxide (TiO2) clusters is investigated. Cl2 gas and its compounds are known for their wide use in industry. However, the high toxicity of the gas forces its users to use sensors to detect its probable leakage. TiO2 is one of the most stable oxides. The surface decoration or surface doping of TiO2 with Pd or other catalysts increases its sensitivity to Cl2, reduces response time, and reduces the temperature needed to best sensitivity to the gas. In the present work, transition state theory is used to simulate the reaction of Cl2 with pristine and Pd-doped TiO2 clusters. The three steps of physisorption, transition state, and chemisorption are described. A comparison of experimental and theoretical results assists the temperature-dependent Gibbs activation energy because of activation entropy. The comparison includes temperature-dependent sensitivity, concentration-dependent sensitivity, and response and recovery times. The evaluation of thermodynamic quantities, i.e., Gibbs free energy and entropy, are needed to perform present calculations using density functional theory. The present work is one of the rare applications of transition state theory in gas sensing.