Appropriate equations for representation of excess thermodynamic properties of moderately dilute solutions are presented with discussions. Illustrative examples are given, based largely on the experimental results of Morita et al. It is shown that the statistical thermodynamic treatments with enumeration of configurations, and with the free volume concept yield the best existing equations. Future developments will depend on realistic enumeration of configurations with models having attractive and repulsive forces. The lack of adequate data on the physical properties, such as the excess volume, electronic configuration, etc., have discouraged further realistic developments.