Recent experimental work on well-defined colloids has shown that in certain systems, colloid stability increases, reaches a maximum and then decreases as the particle size increases. This finding, in apparent violation of D.L.V.O. theory predictions, is shown to be understood in terms of an energetic rather than a kinetic criterion of colloid stability. By incorporating both primary and secondary minima coagulation, and by the construction of coagulation domain diagrams, these “anomalous” particle size effects are shown to be in agreement with D.L.V.O. theory. The theory is applied to recent studies on monodisperse latex colloids of varying particle size.