We show that flow-induced coalescence is facilitated when the molecular weight of the matrix fluid becomes large enough. For a system that consists of polybutadiene drops in a suspending fluid of polydimethylsiloxane, and a viscosity ratio of the drop to the suspending fluid of 0.5, the critical molecular weight is found to be approximately 10(5). For the range of molecular weights and drop sizes studied here, the shear rates remain low enough that bulk viscoelastic effects are negligible. Instead, we hypothesize that coalescence is facilitated because there is slip at the boundaries of the thin film between the drops, and this facilitates its drainage to a point where the film ruptures. This suggestion is tested in two indirect ways. First, we extend the film drainage analysis to include slip and show that the analysis seems to be qualitatively consistent with the. data. Second, we carry out additional experiments in which the interface of the drops is covered with a "compatibilizer." These experiments show that the molecular weight effect is largely suppressed, due apparently to the fact that the copolymers span the interfacial zone and are entangled with the bulk polymers on the two sides of the interface, thus largely eliminating the possibility for slip. (C) 2003 The Society of Rheology.