A theoretical investigation of the reactions of Criegee intermediate (0) with water trimer and tetramer at B3LYP/6-311+G(2d,2p) level of theory was performed. Eight different configurations of complexes CI...(H2O)(3) and eight of CI...(H2O)(4) were found. Their subsequent reactions were considered, and the corresponding activation barriers were also determined. Atmospheric water cluster concentrations were estimated at MP2/CBS//MP2/aug-CC-pVTZ level of theory with counterpoise correction. The relative reaction rates were calculated for all studied reaction pathways. Our results indicate that reactions with larger water clusters could be important under certain atmospheric conditions. The most kinetically favorable pathway is the CI with water dimer reaction. (c) 2005 Elsevier B.V. All rights reserved.