In order to get more knowledge about the fundamentals of solvent-free slack wax de-oiling by melt crystallization, investigations on multicomponent paraffin waxes have been performed, which included X-ray powder diffraction and optical microscopy. The tested neutral oil and medium slack waxes consisted mainly of n- and iso-paraffins with different chain lengths. The results suggest that only n-paraffins formed orthorhombic crystals at room temperature with c-axes corresponding to the average chain length of the paraffin waxes. The crystals were surrounded by less crystalline, rather amorphous, solid phase containing mainly iso-paraffins. The typical solid-solid transition of crystal lattice in neutral oil slack waxes from orthorhombic to hexagonal system at temperatures below melting point increases the mobility of the low melting components. This is the key for the applicability of the melt crystallization process. Investigations on sweating suggest a strong influence of the n-paraffins content on mechanical stability of crystal structures and on the separation efficiency of sweating. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.