Technical report. From Abstract : "The MBr3 + M phase diagrams for lanthanum, cerium, and praseodymium are presented. Lanthanum and cerium exhibit simple solutions of metal, with 14 and 12 mole % dissolved at eutectic temperatures of 728 and 687°, respectively. The compound PrBr2.38 is found, melting incongruently to a solution of 16% Pr in PRBr3 and Pr(s) at 601°. For praseodymium the cryoscopic behavior of dilute solutions of metal is consistent with the primary formation of the Pr2+ solute. But with lanthanum and cerium the results are anomalous, as the cryoscopic analyses indicate 1.6-1.7 and 2.0 new particles (cation basis) per metal dissolved instead of ~3."