The influences of molecular weight and LLDPE comonomer type on the heterogeneity (immiscibility) of Ziegler-Natta LLDPE and LDPE blends are investigated with rheological methods. Dynamic and steady shear measurements were carried out in a Rheometrics Mechanical Spectrometer 800. Blends of low-M-w (<10(5))LLDPE (butene) and LDPE are likely homogeneous and miscible as revealed by the dependence of their eta' on blend composition at 140degreesC. Blends of high-M-w (10(5))LLDPE (butene) and LDPE mixed and tested at 190degreesC were only partially miscible; heterogeneity and immiscibility was likely to occur around the 50/50 composition and in the LDPE-rich blends. Blends were likely miscible in the LLDPE-rich range. Increasing the LLDPE branch length (comonomer) from butene to octene slightly increased the miscibility of LLDPE/LDPE blends. It is suggested that the molecular order in polyethylenes (see Hussein and Williams (1999) J Non-Newtonian Fluid Mech 86:105-118; (1998) Macromol Rapid Commun 19:323-325) and mismatch of the molecular conformations of different polyethylene structures provide explanations for the immiscibility of polyethylenes. Agreement was observed between the measured G'(omega) and G"(omega) and theoretical predictions of Palierne and Bousmina-Kerner models, which are based on two-phase emulsion behavior.