Measurements of dissolution enthalpies in heptane at ambient temperature were carried out by mixing calorimetry on the n-alkanes going from C20H42 to C36H74. These dissolution enthalpy values increase linearly as a function of the carbon atom number. Furthermore, they reveal an alternating effect between odd- and even-numbered n-alkanes and highlight that the mixing of an n-alkane in the liquid phase with heptane is an athermal process: the variation of its enthalpy from the dissolution temperature up to its melting temperature corresponds to its dissolution enthalpy in heptane.