A theory of He3-He4 mixtures is developed, in which He3 is represented by a nondegenerate Fermi gas in a potential well, and He4 by an assembly of Bose-particles another potential well, each Bose-particle being considered to have a much larger mass than a He4-atom and to have an energy gap between the ground state and the lowest excited state. Agreement between theory and experiment concerning the transition temperature and the vapour pressure in solution is satisfactory, if the energy of excitation is assumed to be proportional to the number density of He4 “;particles” in solution. The increase of the velocity of second sound in solution can be accounted for based on this theory under the condition that He3 particles partake in the motion of the normal part particles, in contrast to Koide nad Usui’s conclusion. The osmotic pressure, the thermomechanical effect, and the specific heat in solution are also discussed. It is also remarked that the analogue, in solution, of Mendelssohn and Chandrasekhar’s experiment would offer the most definite answer to the question which of the two theories is more tenable, de Boer and Gorter’s or the present author’s. Second sound for dilute solutions near the absolute zero of temperature is also discussed, and it is concluded that the square of the velocity should be there proportional to T and the proportionality coefficient is roughly equal to R/3. Theory and experiment are in general agreement also in this case.