2This chapter is a treatment of the behavior of dilute mixtures and solutions near the solvent’s critical point. Although these mixtures form only a modest subset of the supercritical mixtures that are used in the supercritical process technology, their interest ranges beyond the confines of chemical technology. Dilute aqueous solutions, for instance, are encountered in the power industry, oceanography, and geology; when these systems are taken to the vicinity of the critical point of steam (647 K and 22 MPa), as they are in practice, the framework of solution physical chemistry, carefully erected during the 20th century, shakes on its foundation; unexpected, often spectacular effects have been reported.