Measurements of the dissolution enthalpies in heptane at ambient temperature were carried out by mixing calorimetry on multialkane synthetic mixtures with an exponentially decreasing distribution of the n-alkane mole compositions, as observed in petroleum cuts. These experimental results and those of literature concerning the pure n-alkane dissolution enthalpies in the same experimental conditions allow us to determine the enthalpy of the formation of one or several crystallized solid solutions of these mixtures and thus, to highlight the influence of the characteristic parameters of the paraffinic distribution (theoretical average chain length, n-alkanes number, monophasic or polyphasic state) on the excess properties in the solid state. Second, this data bank permits us to test the predictive capacities of the UNIQUAC thermodynamic model. After modifying the expression of the interaction energy lambda(ij) to take into account the effect of the size difference between the n-alkanes of the mixture and the internal disorder induced by the n-alkane distribution length, this model gives very good results for the prediction of the excess enthalpy of complex multialkane samples.