The Liquid-liquid miscibility temperatures as a function of composition and deuterium substitution have been determined experimentally for the binary systems formed by acetonitrile or nitromethane with decanol and tetrachloroethylene including their deuterated forms. The following deuterated compounds were used: acetonitrile-d(3), nitromethane-d(3) and decanol-d(1). It appears that deuteration of the methyl group in acetonitrile or nitromethane leads to the significant upward shift of the upper critical solution temperature, This effect is discussed on the basis of the intermolecular interaction emphasizing the role of the strong dipole-dipole interaction between acetonitrile (or nitromethane) molecules, Deuterium substitution in OH group of decanol generates a weak downward shift of the upper critical solution temperature. It is believed that in this particular case the better miscibility upon deuterium substitution results from hydrophobic action of the large alkyl group obscuring the OH(D) group and restricting hydrogen bonding formation with this group.