The molar enthalpies of mixing, ΔmixHm in the binary liquid systems TbCl3 - MCI ( M = Li, Na, K, Rb, Cs ) have been measured with a Calvet - type high temperature microcalorimeter over the entire composition range with an accuracy of about 6%. Mixing of the two liquid components was achieved by using the “break-off ampoule technique”. All the investigated systems show negative enthalpies of mixing with a minimum value of approximately -2.4, -9.9, -20.5, -26.0 and -28.8 kJ mol-1, for M = Li, Na, K, Rb and Cs, respectively occuring at nearly the same molar fraction of TbCl3, xTbCl3 ≈ respectively, at XTbCl3 ~0.4. The molar enthalpies of formation ΔformHm (3MCl,TbCl3,l) for M = Li, Na, K at 1109 K and for M = Rb and Cs at 1175 K, arising from the reaction 3MCl(l) + TbCl3(l) = (3MCl,TbCl3)(l) are found to be -9.6, -36.1, -72.6, -87.4 and -97.6 kJ mol-1, respectively. The least-squares coefficients A, B, C, D in the equation λ (kJmol-1)= A + Bx + Cx2 + Dx3 where λ is an interaction parameter are also reported.