The present work is part of the thermodynamic research performed on the LnCl3-MCl systems (where Ln is lanthanide and M is alkali metal). The molar enthalpies of mixing DELTA(mix)H(m) in the NaCl-PrCl3 and KCl-PrCl3 liquid binary systems were measured at 1122 K over the whole composition range under argon at atmospheric pressure, with an accuracy of about 6%. The apparatus used was a Calvet-type high-temperature microcalorimeter and mixing of the two liquid components was obtained by the break-off ampoule technique. In the systems, the enthalpies of mixing values are negative, with a minimum value of about - 7 and - 16 kJ mol-1 for NaCl-PrCl3 and KCl-PrCl3, respectively, at x(PrCl3) almost-equal-to 0.4. These values are very similar to those relative to the systems MCl-LnCl3 previously investigated. The molar enthalpies of formation DELTA(f)H(m)(K3PrCl6, 1, 1122 K)/kJ mol-1, DELTA(f)H(m)(Rb3PrCl6, 1, 1122 K)/kJ mol-1 and DELTA(f)H(m) (Cs3PrCl6, 1, 1122 K)/kJ mol-1, according to the reaction 3MCl(1) + PrCl3 (1) = M3PrCl6(1), are equal to - 55.9, - 66.4, and - 80.4 kJ mol-1, respectively, and are almost identical (in the limits of experimental uncertainty) to those of the compounds M3LnCl6 (where M is K, Rb, Cs, and Ln is La, Ce, Nd) previously investigated. The least-squares coefficients A, B, C, D and E of the equation for the interaction parameter lambda = A + Bx + Cx2 + Dx3 + Ex4 (in kJ mol-1 are presented.