A combined WAXD and d.s.c. study on blends of a series of commercial high density and low density (LD) polyethylene (PE) samples rapidly crystallized from the melt is presented. The melting curves of the materials are extensively analysed and compared to those exhibited by the individual components. The results, besides being indicative of a full compatibility of the two components, allow one to distinguish between a blend and an LDPE having both the same average value of branching. However, other structural parameters such as unit cell volume, macroscopic density or crystallinity, that are proved to be an exact average of the components, are unable to distinguish between the two materials. The result is applied to a series of commercial bimodal PEs.