It has been traditional each year to include in this volume a section 011 thermochemistry and on the thermodynamic properties of materials. Since thermodynamics is so much a part of all the special areas of physical chem istry, completely reviewing this subject is tantamount to reviewing all of physical chemistry. Researches involving thermodynamics are being carried out at such a rapid pace that it is no longer possible in one short chapter to review the entire field, unless the content is limited to an extensive bibliog raphy. However, such compilations of references a re r eadily available on a yearly basis. The International Union of Pure and Applied Chemistry through various commissions and subcommissions [Commission on Thermo dynamics and Thermochemistry, Commission on High Temperatures and Refractories (1)] provides a number of compendia of current research and research in progress. In addition there are compilations, reviews and correla tions (2, 3) of thermodynamic data which appear regularly. Therefore, in this chapter a selection of subject matter has been made. This review has been limited to a single topic, the thermodynamics of mixtures at low temperatures, an area in which there has been some considerable progress of late and which has not been covered in the Annual Review in recent years. Further, in this chapter we shall deal exclusively with mixtures which show appreciable deviations due to quantum effects from what might be called classical behavior. A useful yardstick for the estimation of these quantum deviations is the De Broglie wavelength, A=h/mv. For a given molecule, when A is of the order of the molecular dimensions, quantum effects wiII be of importance. Obviously, then, these departures from classical behavior should manifest themselves primarily in the thermodynamic properties of the lightest elements and their isotopes at very low tempera tures. These deviations arise from large differences in zero point energies. quantum statistical effects and diffraction effects associated with the wave-