We employ rheological measurements of polyethylene blends in the melt and solid state, together with thermal analysis, to infer phase behavior. Partial-miscibility in the melt is characterized by use of the double-reptation model to define the complex modulus of the continuous phase for input into the emulsion model for the blend; this approach introduces a new fitting parameter, the fraction of the minor component contained in the continuous phase. The results on binary systems suggest the use of HDPE as a compatibilizer for LLDPE/LDPE blends, apparently creating a fully miscible ternary system.