The enthalpies of mixing of more than 70 A(p)B-AC molten salt systems, containing ions with different valences, have been found in literature. (B = rare earth or alkaline earth metal; C = alkali metal; A = halide.) Experimental heat of mixing data were fitted against composition to an analytical equation deduced from the Surrounded Ion Model (SLM). The limiting molar partial enthalpy of mixing of A(p)B in AC (H-infinity(A(p)B)) or of AC in A(p)B (H-infinity(AC)), were thus calculated. The evaluation of these limiting molar partial enthalpies for a series of systems, for example: the rare earth chlorides alkali chlorides mixtures, is related to the charge and to the radius of the involved ions. This work made it possible to propose for these limiting partial enthalpies the following analytical expressions: H-infinity(A(p)B) = alpha Delta IP + beta/r(C+) + gamma or H-infinity(AC) = alpha'Delta IP + beta'/r(Bp+) + gamma' where Delta IP is the difference of the ionic potentials of the Bp+ and C+ ions: Delta IP = (p/r(B) -1/r(C)) and alpha, beta, gamma, alpha', beta', gamma', constants deduced from our calculations.