Summary: We consider a stochastic particle model for coagulating particles, whose free motion is Brownian, with diffusivity given by Einstein’s law. We present in outline a derivation from this model of a spatially inhomogeneous version of M. van Smoluchowski’s [Drei Vorträge über Diffusion, Brownsche Bewegung und Koagulation von Kolloidteilchen. Phys. Z. 17, 557–559 (1916)] coagulation equation. Some analytic results on existence, uniqueness and mass conservation for the limit equation are also presented.