The thermal reaction for the solid mixture system with a 1:1 molar ratio of hydrated lanthanoid(III) chloride and dibenzo−18-crown−6 (dbc) has been investigated by TG—DTA at a nitrogen pressure of 20 mmHg. The reaction was composed of dehydration, melting, complexation, and decomposition. Three patterns for the dehydration in the six systems have been exhibited through the reaction. (1) La—dbc system: all dehydration steps completed prior to the melting—complexation step. (2) Nd—, Sm—, and Eu—dbc systems: the last dehydration took place just after the melting—complexation step. (3) Dy— and Er—dbc systems: the last water was kept until the complex initiates decomposition. Under the fusion state of dbc, about 66–87% mixture formed the complex with a 1:1 stoichiometry. Enthalpy changes for the complexation, estimated from the DTA analyses under excess amounts of the rare earth salt, decrease with the lanthanoid contraction except for the Er—dbc: 31.0, 24.9, 20.3, 15.5, 14.9 and 15.7 kJ mol −1 for the La, Nd, Sm, Eu, Dy, and Er systems, respectively. With the increasing order of lanthanoid(III) radius, the stability of the complex formed has enhanced. The hydrated water plays an important part in the complex formation: the water molecule seems to be kept stable in the complex when the ion size of the lanthanoid(III) decreases.