Abstract Dynamic two-dimensional Fourier transform infra-red ( FTi.r.) spectroscopy has been used to study the nature of the interphase in low-density polyethylene (LDPE). A two-dimensional correlation analysis on the dynamic spectra indicates that neat LDPE is comprised of three regions: an ordered crystalline region, a disordered, liquid-like region, and a crystal/amorphous interfacial region. The 1468 cm −1 peak in the methylene bending region is assigned to all- trans structures which primarily reside in the interfacial region. A variety of LDPE samples with different additives are used in order to determine how the additives influence the dynamic mechanical properties of each morphological phase. It is found that talc associates with the crystallites, ethylene vinyl acetate is distributed in all three morphological phases of the LDPE, and pyrene is associated only with the non-crystalline regions.