The molar enthalpies of mixing DELTA(mix)H(m) in the PrCl3-CaCl2 and NdCl3-CaCl2 liquid binary systems were measured at 1073 K over the whole composition range under argon at atmospheric pressure. The apparatus used was a Calvet-type high-temperature micro-calorimeter and mixing of the two liquid components was obtained by the break-off ampoule technique. In both systems the plots of enthalpies of mixing versus composition were "S-shaped" curves. The enthalpy of mixing values are positive for NdCl3- and PrCl3-rich compositions, and negative for other compositions. These values are very small (from about -200 to about 180 J mol-1). In the PrCl3-CaCl2 and NdCl3-CaCl2 liquid binary systems, DELTA(mix)H(m) almost-equal-to 0 at x(PrCl3) almost-equal-to 0.6 and at x(NdCl3) almost-equal-to 0.4, respectively. These results are discussed in terms of equilibrium between the LnCl3 (where Ln is Pr, Nd) polymers and anionic complexes as a function of the composition of the melts.