An equilibrium model for agglomeration in polycrystalline thin films which considers the energy balance between the grain boundary energy and both surface and substrate interface energies is presented. It predicts that small grain size, low grain boundary energy, and high film surface and interface energies should promote resistance to agglomeration, and shows that the substrate‐film interface can play a significant role in the process. It also predicts a critical grain size limiting formation of a discontinuous island structure. This easily calculable value is significantly smaller than that found in previous modeling. The critical grain size, the importance of the substrate interface, and some of the assumptions are shown to be consistent with transmission microscope observations of TiSi2thin films deposited on Si substrates.