Abstract.The physical process of coagulation or coalescence of particles is often modelled by Smoluchowski's coagulation equation, an infinite system of nonlinear differential equations governing the binary interactions of particles of different sizes. One of the physical assumptions underlying the coagulation equation is spatial homogeneity, in particular, the assumption that coagulation of particles is governed only by particle type or size. The physical shape of the cloud of particles, as well as the relative position of particles of various type, are ignored.One way to model spatial inhomogeneities is to consider several localized clusters of particles distributed within the cloud, with each cluster being a microcosm of coagulating particles. That is to say that coagulation occurs among particles within a cluster but not among particles from different clusters.Interaction between clusters is then accommodated by allowing for a migration, or drift, of particles between clusters. In this article, we model this cloud of localized clusters by a directed graph, with a vertex located at each cluster of particles and a directed edge between vertices representing the migration of particles between clusters, and investigate the effect of the shape of the graph on the properties of the solutions and their evolution.For the case of a constant coagulation kernel, we prove the existence, uniqueness, and global stability of the solutions, as well as mass conservation, in this more general setting.We also give examples of some of the novel features arising from the structure of the graph, such as the possibility of oscillatory behavior, and the much greater degree of control over the creation of large particles, a problem of some importance in practical applications.