A number of differential scanning calorimetry (DSC) experiments were performed to investigate the melting behavior of blends of low density polyethylene (LDPE) and high density polyethylene (HDPE). Results revealed three endothermic peaks which suggested that, upon fast cooling, a "co-crystalline" phase is formed. The same feature was found using Temperature Rising Elution Fractionation (TREF), which fractionates semi-crystalline polymers based on relative crystallizability of molecules, Isolation of the "co-crystallized" fraction was possible using TREF in a preparative mode. Analytical TREF and DSC were then used to identify polymeric components in the "co-crystalline" phase.