A hypothesis (theory) has been formulated for steric stabilization during simple shear flow of discrete domains of one polymer A in a continuous phase B by block copolymer addition. A limiting solution found when the molecular weights of the components are high predicts preformed small drops stabilized by block copolymers at the interfaces are stable below a critical shear stress τcrbut unstable above τcrInterfacial placement in well dispersed blends of PS/PMMA/PS-b-PMMA was deduced by interfacial tension measurement, structure development studies, and viscosityshear rate studies with and without block copolymers. The experimentally determined critical shear stresses show relatively poor reproducibility (SD ± 10%). For measurements at shear stresses larger than the critical values, interesting time effects are observed, whereas scattering signals are constant at shear rates lower than τcrThe hypothesis is not rejected when compared with experimental data from light-scattering measurements during flow.