The mechanisms for reactions of H, HO, and Cl with HOClO3, important elementary processes in the early stages of the ammonium perchlorate (AP) combustion reaction, have been investigated at the CCSD(T)/6-311+G(3df,2p)/ /PW91PW91/6-311+G(3df) level of theory. The rate constants for the low-energy channels have been calculated by statistical theory. For the reaction of H and HOClO3, the main channels are the production of H2 + ClO4 (k1a) and HO + HOClO2 (k1b); k1a and k1b can be represented as 1.07 × 10 -17 T 1.97 exp(-7484/T ) and 6.08 × 10 -17T 1.96 exp(-7729/T ) cm3 molecule -1 s-1, respectively. For the HO + HOClO3 reaction, the main pathway is the H2O + ClO4 (k 2a) production process, with the predicted rate constant k 2a = 1.24 × 10 -8 T -2.99 exp(1664/T ) for 300-500 K and k2a = 1.27 × 10-19 T 2.12 exp(-1474/T) for 500-3000 K. For the Cl + HOClO3 reaction, the formation of HOCl + ClO3 (k3a) andHCl + ClO4 (k3b) is dominant, with k3a = 1.33 × 10-12 T 0.67 exp(-9658/T ) and k3b = 1.75 × 1016 T 1.63 exp(-11156/T ) cm3 molecules-1 in the range of 300-3000 K. In addition, the heats of formation of ClO3 and HOClO3 have been predicted based on several isodesmic and/or isogyric reactions with ΔfH 0o (ClO3) = 47.0 ± 1.0 and ΔfH0o (HOClO3) = 5.5 ± 1.5 kcal/mol, respectively. These data may be used for kinetic simulation of the AP decomposition and combustion reaction.