Theoretical investigations are carried out on the mechanism, kinetics, and thermochemistry for the reaction between pyridine and atomic chlorine with use of the hybrid density functional model BB1K, BHandHLYP, and the ab initio MP2 method. Both addition to and hydrogen abstraction reaction from all the potential sites are considered. Barrier heights and thermochemistry for all the possible addition and abstraction processes are calculated for the first time. This helps to identify the most reactive reaction channels in different temperature ranges. The DFT-based reactivity descriptors are used to elucidate the site selectivity for the addition and abstraction reactions. The results obtained from the BB I K method are found to be in reasonably good agreement with the available experimental results.