We consider the effects of adding a PIB-PDMS diblock copolymer as a compatibilizer in model blends composed of polyisobutylene (PIB) and polydimethylsiloxane (PDMS). The ratio of PIB to PDMS was varied from 20:80 to 80:20 and, hence, these blends are dubbed "concentrated" blends. Most experiments were conducted on blends containing 0.01 or 0.1 wt. % compatibilizer. All blends had a droplet-matrix morphology, with the minor phase being the drop phase; thus, phase inversion occured when PIB and PDMS were in a 50:50 ratio. Previously, we have studied effects of compatibilizer in "dilute" blends with PIB and PDMS in a 10:90 ratio. Much of the theological behavior of the concentrated blends studied here is found to be qualitatively similar to that of dilute blends: compatibilizer increases the terminal complex viscosity, the terminal relaxation time, steady shear viscosity, and the ultimate recovery upon cessation of shear. However, there are two noteworthy differences. The first is that in blends in which PIB forms the continuous phase, the compatibilizer can suppress coalescence of the PDMS drops. Therefore, theological properties that depend on drop size, e.g., relaxation time or ultimate recovery, are correspondingly affected. Second, the compatibilizer increases the viscosity, especially the terminal complex viscosity, of the concentrated blends far more than it does in dilute blends. This can be interpreted in terms of a partial immobilization of drop surfaces by the compatibilizer. Indeed, in blends with PIB as the continuous phase, the viscosity is only slightly lower than of a suspension of rigid spheres, suggesting that the compatibilizer immobilizes the interface almost completely. (c) 2007 The Society of Rheology.