The linear viscosity of m-LLDPE/LDPE blends is adjusted to a free volume model, conceived for miscible blends. A deviation from the model is observed at 47.5% m-LLDPE/52.5% LDPE blend, suggesting immiscibility in the molten state at this composition. Time-temperature superposition method is used to confirm miscible and immiscible cases. The effect of miscibility on practical theological features is analysed using extrusion rheometry. The results indicate a core-sheet morphology in the immiscible blend, as the less viscous LDPE encircles the m-LLDPE phase. Miscible blends with a high LDPE content and immiscible 47.5% m-LLDPE/52.5% LDPE blend, show 'melt fracture', but not 'sharkskin'. The latter is observed in miscible blends of a high m-LLDPE content. 'Sharkskin' is postponed in 87.5% mLLDPE/12.5% LDPE blend, a result which is associated to the elongational viscosity enhancement, due to the presence of long chain branches. The correlation between melt spinning and blown film extrusion results is investigated, showing evidences of the technical limitations caused by immiscibilty. (c) 2005 Elsevier Ltd. All rights reserved.