Abstract: The coordination reaction of lanthanum nitrate with 18-crown-6, dicyclohexyl-18-crown-6 and 2,3-benzo-11-methyl-18-crown-6 in acetonitrile was studied. The reaction enthalpies and stability constants were obtained by home-made titration calorimeter and computer program. We demonstrated that the reaction is exothermic on condition that the lanthanum nitrate was dehydrated rigorously. Supplementary experiments showed that the reaction heat is dependent obviously on water content in lanthanum nitrate. Because lanthanum (III) ion has strong hydrophilic ability and large hydration heat, large energy consumption is necessary when crown replaces the coordinated water. Therefore, if the lathanum nitrate has not been dehydrated rigorously, the reaction heat measured may be small negative or positive value.