The reaction between the HO radical and (H(2)O)n (n = 1, 3) clusters has been investigated employing high-level quantum mechanical calculations using DFT-BH&HLYP, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311 + G(2df,2p), aug-cc-pVTZ, and aug-cc-pVQZ basis sets. The rate constants have also been calculated and the tunneling effects have been studied by means of time-dependent wavepacket calculations, performed using the Quantum-Reaction Path Hamiltonian method. According to the findings of previously reported theoretical works, the reaction between HO and H(2)O begins with the formation of a pre-reactive complex that is formed before the transition state, the formation of a post-reactive complex, and the release of the products. The reaction between HO and (H(2)O)(2) also begins with the formation of a pre-reactive complex, which dissociates into H(2)OaEuro broken vertical bar HO + H(2)O. The reaction between HO and (H(2)O)(3) is much more complex. The hydroxyl radical adds to the water trimer, and then it occurs a geometrical rearrangement in the pre-reactive hydrogen-bonded complex region, before the transition state. The reaction between hydroxyl radical and water trimer is computed to be much faster than the reaction between hydroxyl radical and a single water molecule, and, in both cases, the tunneling effects are very important mainly at low temperatures. A prediction of the atmospheric concentration of the hydrogen-bonded complexes studied in this work is also reported.