Annealed interfaces between high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) and some ethylene-co-vinyl acetate (EVA) and ethylene-co-acrylic acid (EAA) copolymers were examined using optical microscopy, scanning electron microscopy (SEM), and electron microprobe analysis. Transcrystalline zones were observed in a polarizing microscope on the copolymer side of the interfaces between EVA copolymers with less than or equal to 18 wt % VA or EAA copolymers with less than or equal to 6.5 wt % AA and HDPE or LDPE when the samples were heated above the melting points of both polymers and allowed to cool slowly to room temperature. The crystallization temperatures in the transcrystalline zones were all above those of the bulk copolymers and, in some cases, above the melting points of the bulk copolymers. Electron microprobe data on EVA-HDPE freeze-fractured interfaces showed a percentage of VA less than that in the bulk copolymer in the region corresponding to the transcrystalline zone on the copolymer side of the interface. The data indicated that polyethylene migrated into the transcrystalline zone. This was confirmed by optical microscopy which showed that some PE oligomers with molecular weights up to 507 were completely miscible with EVA samples having VA contents up to 18 wt %. Also, calculations using Flory-Huggins theory indicated that copolymers of low wt % VA or AA should be miscible with low molecular weight PE at elevated temperatures. Optical microscopy of EAA or EVA interfaces with PP showed the formation of influxes of copolymer into the PP side of the interface, resulting in a mechanically interlocked interface.