Three ethylene-butene copolymers (two metallocene-based and one conventional) having different composition distributions were blended with low density polyethylene (LDPE) in a wide proportion range. The miscibility of the blend melts was studied with the rheological method. It is found that zero-shear viscosity of all the blends does not obey the logarithmic additivity rule, indicating the immiscible state of the blend melts. The blends were rapidly quenched from the melts and co-crystallization phenomena were investigated by differential scanning calorimetry (DSC). The obtained results show that composition distribution has a great influence on co-crystallization. In the blends of the conventional ethylene-butene copolymer, which has a broad composition distribution, part of the co-crystals may segregate from the crystals of both LDPE and ethylene copolymer and exist as a separated population. This suggests that there exist three phases in the melts. In contrast, the blends of the metallocene-based ethylene-butene copolymers with a narrow composition distribution are only composed of two phases together with some exchange between the two components. It is also observed that broader composition distribution leads to more ethylene-butene copolymer transferred into LDPE. (C) 2001 Elsevier Science Ltd. All rights reserved.