Conformational characteristics of poly(trimethylene sulfide) (PTMS) have been investigated by a isomeric state analysis of ab initio molecular orbital calculations and H-1 and C-13 NMR experiments for a monomeric model compound, 1,3-bis(methylthio)propane, and the characteristic ratio and dipole moment ratio of the unperturbed PTMS chain. Both C-S (the first-order interaction energy, -0.30 kcal mol(-1)) and C-C (-0.58 kcal mol(-1)) bonds prefer the gauche conformation. For comparison, conformational analysis of the corresponding polyether, poly(trimethylene oxide), has also been carried out. The C-O (+0.97 kcal mol(-1)) and C-C (-0.47 kcal mol(-1)) bonds show trans and gauche preferences, respectively. These results are consistent with the conformational stabilities predicted by the natural bond orbital analysis on model compounds of poly(ethylene sulfide) (PES) and poly(ethylene oxide). Without the strong (SS)-S-... repulsion, therefore, the C-C bond adjacent to the C-S bond exhibits its inherent gauche preference. Without the (C-H)O-... attraction, the ethereal C-C bond is subject to the attractive gauche effect. For representative polysulfides and polyethers, relationships between conformations in the O and crystalline states and thermal properties have been investigated. Even in the 0 and crystalline states, the polymers mostly keep the conformational preferences found for their small model compounds; as an exception, poly(propylene sulfide) and poly(propylene oxide), having a methyl side chain, crystallize to adopt the metastable all-trans conformation. On average, the configurational entropy of the polymers amounts to 80-90% of entropy of fusion, and the configurational energy change between crystalline and molten states accounts for ca. 30% of enthalpy of fusion. Of the polymers investigated here, PES has an exceptionally high melting point; the dipole-dipole interaction in the PES crystal was evaluated to be -1.0 kcal mol(-1) (ca. 30% of the enthalpy of fusion), thus being the source of the thermostability.