Liquid-liquid miscibility temperatures as a function of composition and deuterium substitution have been experimentally determined for binary mixtures of aniline and cyclohexane and their various deuterated forms: aniline-d(2), aniline-d(5), cyclohexane-d(12). In all cases studied, the deuteration leads to the upward shift of the upper critical solution temperature. This effect is discussed in the frame of the theory of the condensed phase isotope effects coupled with the phenomenological g(E) thermodynamic model of solutions. (c) 2005 Elsevier B.V. All rights reserved.