The conformational equilibrium in neat liquids consisting of small chain molecules, n-butane and 1.2-dichloroethane, is studied by means of a RISM-type integral equation theory formulated by Yoshida and co-workers [T. Munakata, S. Yoshida, F. Hirata, Phys. Rev. E 54 (1996) 3687]. Conformational shift toward the gauche isomer was observed upon transferring from gas to liquid phases in the both fluids in harmony with experiments. The equilibrium constant for 1,2-dichloroethane shows very small temperature dependence. The pressure dependence of the equilibrium constant indicates that the gauche isomer is in favor at higher temperature. The theoretical results are consistent with the experimental observations. (c) 2005 Elsevier B.V. All rights reserved.