A linear polyethylene (LPE) has been blended with each of three ethylene-co-4-methyl pentene-1 (E4MP1) copolymer fractions of differing branch content (2.0, 4.3, and 6.1 mol% 4-methyl pentene-1). These copolymers have Y-shaped branches, each containing four carbon atoms. The blends were studied using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) following experimental procedures that have been extensively used to investigate blends of LPEs with copolymers with alkane (linear) branches. When the copolymer is lightly branched, LPE/E-4MPl copolymer blend systems give, to a first approximation, morphology maps similar to those found for linear/branched polyethylene systems in which the branched polyethylene has linear alkane branches. Single morphologies are found for high LPE contents, but there are double morphologies (regions of thick crystals well separated in a matrix of thin crystals) at low LPE contents after quenching from within a restricted temperature range. As the branch density of the copolymer increases, the same changes in morphology map are seen for the LPE/E4MPl copolymer blend systems as for lineadalkane branched copolymer systems. The final morphology map, characteristic of a reasonably heavily branched copolymer, is reached at a lower branch density (near 6.1 mol%) for the LPE/E4MPl copolymer blend systems than for LPE/linear alkane branched copolymer systems. For LPE/E-4MP1 copolymer blend systems, as for linearlbranched polyethylene systems in which there are alkane branches, TEM is more sensitive than DSC detecting changes from single to double morphology.