Various properties (such as optimal structures, structural parameters, hydrogen bonds, natural bond orbital charge distributions, binding energies, electron densities at hydrogen bond critical points, cooperative effects, and so on) of gas phase ethanol–(water)n (n = 1–5) clusters with the change in the number of water molecules have been systematically explored at the MP2/aug-cc-pVTZ//MP2/6-311++G(d,p) computational level. The study of optimal structures shows that the most stable ethanol-water heterodimer is the one where exists one primary hydrogen bond (OH…O) and one secondary hydrogen bond (CH …O) simultaneously. The cyclic geometric pattern formed by the primary hydrogen bonds, where all the molecules are proton acceptor and proton donor simultaneously, is the most stable configuration for ethanol–(water)n (n = 2–4) clusters, and a transition from two-dimensional cyclic to three-dimensional structures occurs at n = 5. At the same time, the cluster stability seems to correlate with the number of primary hydrogen bonds, because the secondary hydrogen bond was extremely weaker than the primary hydrogen bond. Furthermore, the comparison of cooperative effects between ethanol–water clusters and gas phase pure water clusters has been analyzed from two aspects. First of all, for the cyclic structure, the cooperative effect in the former is slightly stronger than that of the latter with the increasing of water molecules. Second, for the ethanol–(water)5 and (water)6 structure, the cooperative effect in the former is also correspondingly stronger than that of the latter except for the ethanol–(water)5 book structure. © 2012 Wiley Periodicals, Inc.