The aim of this work was to investigate the influence of morphology evolution in immiscible blends of Polyisobutene/(Polydimethylsitoxane) on their rheological properties both in steady and transient states under linear and nonlinear flow regimes. The focus was made on the region below the critical shear rate of drops breakup. Such critical shear rate was determined experimentally through a new route using small angle light scattering technique. Interfacial shear stress, first normal stress difference and shear viscosity of the blend were measured in steady shear regime below the critical shear rate and the obtained results were compared to the predictions of the model of Yu et at. [Yu, W., M. Bousmina, A Grmela, and C. Zhou, "Modeling of oscillatory shear flow of emulsions under small and large deformation field," J. Rheol. 46, 1401-1418 (2002a) and Yu, W., M. Bousmina, M. Grmela, J. F. Palierne, and C. Zhou, "Quantitative relationship between rheology and morphology in emulsions," J. Rheol. 46, 1381-1399 (2002b)]. Finally, the influence of the initial morphology on the rheological material functions of the blend was investigated during shear rate sweep-up and shear rate sweep-down tests. The obtained results showed a nice hysteresis that was inferred to morphology development during flow. (c) 2007 The Society of Rheology.