As a continuation of the investigation on the thermal conductivity of solids and liquids, several problems are discussed in this paper. Mixtures of two pure organic liquids have excess thermal resistance over the mean value of those of the components, as has been shown by Riedel experimentally. This is shown to be primarily due to the mass difference between the component molecules. Secondly, it is pointed out that an isotope effect resulting from the mass difference among isotopic atoms plays a role in the restriction of the heat conduction in crystals at very low temperatures, which has been overlooked until now. Finally, the anharmonicity of vibration in solids which constitutes the fundamental mechanism for the establishment of equilibrium among the lattice phonons as well as for the finiteness of the conductivity contributes to the dissipation of the energy of any forced vibration as heat. This effect is estimated numerically and expressed in terms of the thermal conductivity and other physical properties, but it is found its influence is rather small in technically attainable frequencies.