Phase segregation behavior in PEs/DOP blends, interactions between PES and DOP, and glass-relaxation transitions of PEs were investigated. FTIR, DSC, and TGA data demonstrated that molecular interactions were present between PEs and DOP. DMA data demonstrated that pure PEs each (except HDPE) exhibited two loss maxima at about -20 and -120 degreesC but the PEs/DOP blends (including the HDPE/DOP blend) yielded one new loss maximum at about -60 degreesC. The glass-relaxation transitions corresponding to the three loss maxima on the DMA curves were designated alpha (-20 degreesC), beta (-60 degreesC), and gamma (-120 degreesC) transitions and were attributed to the relaxation of the amorphous phases in the interlamellar, interfibrillar, and interspherulitic regions, respectively, based on DMA, WAXD, SAXS, and POM measurements. The controversial T-g values of PEs and their origin were thus clarified in this study.