The melt miscibility of two series of poly(ethylene-r-ethylethylene) (PEExx) polymers with different percentages (xx) of ethylethylene (EE) repeat units was examined with small-angle neutron scattering (SANS). The first series consisted of comb/linear blends in which the first component is a heavily branched comb polymer (B90) containing 90% EE and an average of 62 long branches with a weight-average molecular weight (MW) of 5.5 kg/mol attached to a backbone with MW = 10.0 kg/mol. The comb polymer was blended with six linear PEExx copolymers, all of which had MW ≈ 60 kg/mol and EE percentages ranging from 55 to 90%; they were denoted L55 to L90, with the number referring to the percentage of EE repeat units. The second series consisted of linear/linear blends; the first component, with MW = 220 kg/mol and 90% EE, was denoted L90A, and the second components were the same series of linear polymers, with MW ≈ 60 kg/mol and various EE compositions. The concentrations investigated were 50/50 w/w, except for the blend of branched B90 and linear L90 (both components were 90% EE), for which 25/75 and 75/25 concentrations were also examined. The SANS results indicated that for the comb/linear blends, only the dB90/L90 blends were miscible, whereas the other five blends phase-separated; for the linear/linear blends, dL90A/L83 and dL90A/L78 were miscible, whereas the other three blends were immiscible. These results indicate that long-chain branching significantly narrowed the miscibility window of these polyolefin blends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 466–477, 2002; DOI 10.1002/polb.10102