The phase behavior and morphology of binary mixtures of a homogeneous polystyrene-block-polyisoprene (SI diblock) copolymer and a polyisoprene (hPI) were investigated using small-angle X-ray scattering (SAXS), light scattering, transmission electron microscopy (TEM), and oscillatory shear rheometry with an aim to elucidate homopolymer-induced microphase separation of a homogeneous block copolymer. For the study, the following polymers were synthesized using anionic polymerization: (i) a symmetric SI diblock copolymer (SI-5/5) having a number-average molecular weight (M-n) of 1.0 x 10(4), and (ii) two hPIs having M-n = 1.14 x 10(4) (PI-11) and 1.41 x 10(4) (PI-14). The results of TEM, SAXS and oscillatory shear rheometry showed that SI-5/5 formed a homogeneous single-phase melt, free from any microphase-separated structure in the temperature range of 40-180 degrees C. Binodal curves were constructed, via cloud point measurement, for both (SI-5/5)/(PI-11) and (SI-5/5)/(PI-14) mixtures, and the morphology of the mixtures with and without macrophase separation of hPI was investigated using TEM and SAXS. The TEM study shows the presence of bicontinuous structure in both mixtures, while the SAXS and rheology studies indicate that the mixtures formed a homogeneous single-phase melt. The above observations lead us to conclude that the bicontinuous structure, as determined from TEM, in the (SI-5/5)/(PI-ll) and (SI-5/5)/(PI-14) mixtures represents frozen composition fluctuations in the disordered phase near order-disorder transition and not the homopolymer-induced ordered structure. An explanation is offered as to why in this study homopolymer-induced microphase separation was not observed. (C) 2000 Elsevier Science Ltd. All rights reserved.