Hydrate effects on the conformations of ethylene oxide oligomers (EO-x, x = 1-8 mers) were examined using quantum chemical calculations (QCC). Conformational analyses were carried out by RHF/6-31G. The models were constructed by locating a water molecule to each ether-oxygen in the structures optimized for non-hydrate oligomers. Hydrate ratio, h (h = H2Omol/O-mol in oligomer), was set from 0 to 1.0. The six type conformations with repeated units of O-C, C-C and C-O bonds were examined. Conformational energy, E (c) (HF), was calculated as difference between the energy of oligomer with water molecules and that of non-hydrogen and/or hydrogen bonding water molecules. Hydrate energies for each conformer, a dagger mu (h) (kcal/m.u., based on E (c) in non-hydrate state), were negative and linearly decreased with the increase of h values, and such effects with the increase of h values were weaken with increasing x values. These results were consistent with our previous results calculated using the permittivity, epsilon (epsilon = 0-80.1), by QCC. In non-hydrate (h = 0), the (ttt) (x) conformers were the most stable independent of x. However, in hydrate states (h = 0.44-0.67), the (tg(+)t) (x) conformers were the most stable independent of x values, and in h = 1, the (tg(+)t)(8) conformer (8-mer) was most stable [a dagger E (c(g)) = -1.3 kcal/m.u., a dagger E (c(g)): energy difference between a given oligomer and the (ttt) (x) oligomer]. These results supported the experimental those based on NMR analyses using dimethoxyethane and triglyme solutions. Molecular lengths (l) of (tg(+)t) (x) , (tg(+)g(-)) (x) and (g(+)g(+)g(+)) (x) conformers having higher x values significantly decreased with increasing h values. Such contraction with hydration, however, was independent of Delta E (c(g)) values of each conformer.