In this study, we investigate the thermodynamics of lanthanide-based coordination polymer molecular alloys. We demonstrate that if lanthanide ions have many chemical similarities, the solubility of homo-lanthanide-based coordination polymers can vary significantly from one lanthanide ion to another. Indeed, we experimentally determine the solubility constants of a series of isostructural homo-lanthanide coordination polymers, with general chemical formula [Ln2(bdc)3(H2O)4]∞ with Ln = La-Er plus Y and where bdc2- symbolizes 1,4-benzene-di-carboxylate. Then, we extend the study to two series of isostructural molecular alloys with general chemical formula [Ln2xLn'2 -2x(bdc)3(H2O)4]∞ with 0 ≤ x ≤ 1 based either on heavy ([Eu2xTb2 - 2x(bdc)3(H2O)4]∞) or light ( [Nd2xSm2-2x(bdc)3(H2O)4]∞) lanthanide ions. We found that whatever the solubility difference of the homo-nuclear compounds is, the configurational entropy is the main driving force of the stabilization of molecular alloys.