The influence of B-B diblock copolymers on coalescence in A:B blends has been studied by rheo-optical measurements and electron microscopy. Divergent criteria and experimental evidence appear in the literature on the block copolymer (BC) molecular weight (M(W)) and volume fraction (phi(bc)) when the requirement is the BC to reside at the interface. In the present study the block chain lengths were chosen shorter than the corresponding homopolymers as a starting point. For selected model systems it was found that symmetrical diblock copolymers with phi(bc) greater than or equal to 1% were most effective for inhibiting coalescence. However, rheo-optical measurements revealed that the stabilization effect is not unconditional during the flow; coalescence is prevented for a time which decreases with increasing shear rate due to removal of the BC away from the interface. The origin of the observed behaviour is discussed based on various mechanisms: shear-induced mutual compatibility between components, squeeze-out/drainage of the interfacial layer, frictional pull-out of BC chains, collision-induced entrapment of BC between interfaces, encapsulation of the BC based on the concept of elastic interfacial curvature.