This paper displays a study of the system associating a commercial multi-paraffinic sample and pure n-eicosane: (C20H42). It has been studied by means of calorimetric and structural analyses. The X-ray analysis of the commercial product performed at room temperature has shown that it forms a single solid-solution of orthorhombic structure isostructural to that previously observed with binary or ternary alkane mixtures. The structural characterizations carried out on molecular alloys whose concentration in C-20 is in range from 0 to 100% have allowed to determine the following phase appearance sequence: beta'/beta' + gamma(0)/gamma(0) where beta' is the solid solution of the wax and gamma(0) is the triclinic structure of C-20. The solid-solid transitions of the molecular alloys have been highlighted by DTA and characterized by X-ray diffraction. Compilation of the results led to the proposal of the isopleth section of the system: multi-alkane product-eicosane. In this system, the crystallization is of eutectic' type and several 'invariant' transformations ('eutectoid' or 'peritectoid' reactions) are reported. (C) 2004 Elsevier Ltd. All rights reserved.