The excess partial molar enthalpies, H(m)E(i), and free energies, G(m)E(i), (where i is methanol or isobutanol) were determined at 20.00-degrees-C in methanol-isobutanol mixtures. The Bhatia-March approach (J. Phys. F, 5 (1975) 1100), taking into account the size difference between the components within the framework of the conformational solution theory, was applied to explain the composition dependence of H(m)E(i) and G(m)E(i). It was reasonably successful : however, it did not seem to work for the other isomeric butanol and methanol mixtures.