Explicit computer simulations have unequivocally demonstrated that water significantly affects the conformational equilibrium of n-butane, favouring the gauche conformation with respect to the trans conformation. In this Letter, it is shown that a theoretical approach, grounded on the basic notion that the fundamental difference among the various n-butane conformations is the solvent-excluded volume they produce, is able to quantitatively reproduce the dependence of the conformational Gibbs energy upon the dihedral angle in both water and carbon tetrachloride. (C) 2010 Elsevier B.V. All rights reserved.