A blend of high-density polyethylene and an elastomeric poly(ethylene-co-1-octene) resin, containing 25 mol % octene and long-chain branching, was phase-separated in the melt under quiescent conditions. After melt flow, the blend had fine globular or interconnected phase morphologies that were interpreted as originating from the various stages of coarsening after liquid-liquid phase separation through spinodal decomposition. It was inferred that the miscibility of the blend was enhanced under melt flow. After cessation of flow, concurrent liquid-liquid and solid-liquid phase separation took place, resulting in the formation of an interpenetrating morphology comprising amorphous polyethylene, copolymer, and crystalline polyethylene. (C) 2001 John Wiley & Sons, Inc.