The hydrates of chlorine monoxide, ClO center dot (H2O)(n), n = 1-6, have been studied by means of DFT (B3LYP) method along with aug-cc-pVTZ basis set. Ab initio (MP2 and CCSD(T)) methods and extension of the basis sets up to quintuple-zeta have been done for monohydrates in order to test the methodology. Two rather different intermolecular interactions have been found, namely: conventional hydrogen bonds and OCl center dot center dot center dot O associations. Here it is demonstrated that both types of interactions participate in the formation of hydrates; the OCl center dot center dot center dot O association however produces always the most stable conformations. Equilibrium geometries and relative energies have been calculated and compared with those obtained by other authors. Calculated binding energies indicate that the ClO radical plays an important role in the stabilization of these small clusters, yielding strong interactions with the water molecules. (c) 2007 Elsevier B.V. All rights reserved.