The mechanisms of radical-molecule reactions between HCO (formyl radical) and O-3 (ozone) have been investigated by using BH&HLYP and QCISD methods with the 6-311++G(3df,2p) basis set. The energetics have been refined with CCSD(T) and QCISD(T) theoretical approaches with the same basis set based on the geometries calculated at the QCISD method. The intermediates of hydrogen-bonded complexes and the critical transition states are also examined with the multireference methods. Two possible reaction pathways containing hydrogen-abstraction and association-elimination processes for the interaction of HCO with O-3 are proposed. Both reaction mechanisms can occur via the prereactive hydrogen-bonded complex, O-3-HCO, with 2.45 kcal/mol stability at the CCSD(T) approach with respect to the reactants; even so, the hydrogen-abstraction mechanism exhibits a lower energy barrier. The rate constants for both processes are also predicted. The total rate constant at 298 K is calculated to be in close agreement with the experimental value of 8.3 x 10(-13) cm(3) molecule(-1) s(-1).