The reactions of HOCl + ClOx (x = 1-4) have been studied theoretically by ab initio quantum chemical and statistical mechanical methods. The structures of reactants, intermediates, products, and transition states were optimized at the MPW1PW91/6-311+G(3df,2p) level of theory, and the potential energy surface of each reaction was refined at the G2M and CCSD(T)/6-311+G(3df, 2p) levels of theory. The most favorable reaction channels are predicted to be Cl-abstraction in HOCl + ClO with a barrier of 18.5 kcal/mol and H abstraction in HOCl + OClO with a barrier of 23.9 kcal/mol. In the HOCl + ClO3 reaction both processes can occur; the barriers of Cl and H abstraction are 16.4 and 17.1 kcal/mol, respectively. In the HOCl + ClO4 reaction, the H abstraction transition state lies below that of the reactants by 1.4 kcal/mol. The rate constants for all low barrier channels have been calculated in the temperature range 200-3000 K by statistical theory. In addition, the rate constant for the reverse of the HOCl + ClO reaction, Cl2O + OH f HOCl + ClO, has been predicted; the result is in good agreement with the bulk of available experimental data.