The phase diagram ofn-C12H26+n-C15H32established by calorimetric and X-ray diffraction analyses exhibits no less than four single phase domains [T12, [O15], [Ri15] and [L], two invariants: a eutectic atT= 258.3 K and a metatectic atT= 263.3 K, and five two-phase domains. From a thermodynamic point of view, an interpretation of the phase diagram is given and an attempt is undertaken to modelise the [RI+ L] equilibrium. The extent in compositionxof this phase as well as the width in temperature of the [RI+ L] eutectic atx= 1/2, are explained in terms of degree of isomorphism εim(RI) and excess Gibbs energy functionsGE,sol(x). Those quantities as well as the melting temperature difference (ΔT) atx= 1/2, are compared and discussed with other binary systems.