The molar enthalpies of mixing (Delta H-mix(m)) in liquid alkali bromide-praseodymium(III) bromide mixtures have been measured with a Calvet-type high temperature microcalorimeter over the entire composition range at 1070 K. All of the melts are characterized by negative enthalpies of mixing with a minimum value shifted toward the alkali bromide-rich compositions that are located in the vicinity of x(PrBr3) similar to 0.4. The magnitude of the mixing enthalpy depends on the ionic radius of the alkali metal, radius of the common halide ion, and the ionic radius of the lanthanide(III) cation.