Some general remarks on rare earths' alloying behaviour and their thermodynamics are reviewed and discussed. The role of systematics is underlined and its usefulness in the prediction and assessment procedures is highlighted. Certain aspects peculiar to rare earth Oil alloy thermodynamics with particular attention to the experimental side are presented and discussed. A short summary is given of the work in progress in this field in the authors' laboratory. New enthalpy data obtained in the investigation of some rare earth alloy systems ((La,Ce)-Ag, (Gd,Tb,Dy)-Au, Y-Al, Nd-Sn, Gd-Sb) are reported and compared with the values relevant to the other rare earths.