The effects of plates and slabs on phase change dynamics have been investigated with convection models. Two complementary methods to simulate plates are used: material property and imposed surface velocity methods with temperature-dependent viscosity. For a wide range of model parameters, plates and slabs exert a significant control on phase change dynamics. As plate length (and hence plate age and convection cell aspect ratio) increases, both the propensity for slab penetration and the mass flux across an endothermic phase change increase. When cold downwellings are stiffened with a temperature-dependent rheology, slab penetration is enhanced, but total mass flux changes little. Plates organize large-scale flow and thermal structure and thereby affect phase change dynamics. As plates become larger, the resulting larger-scale structures are influenced less by endothermic phase changes, thus reducing the degree of layering. A model showing completely layered convection for a plate of unit length becomes unlayered when the plate is 3 or 5 times longer. For a given Clapeyron slope, the proportion of time for slab penetration increases from zero for cases with small plates to more than 0.5 for cases with large plates. The degree of layering, plate velocity, and mass flux are controlled by large-scale structures, while slab penetration may be more related to small-scale features. Therefore, whether or not subducted slabs penetrate the phase change may not necessarily indicate that convection is entirely layered or entirely unlayered. The episodicity of convection induced by an endothermic phase change strongly depends on plate length, rheology, and Clapeyron slope. A large plate and a stiff slab both weaken the episodicity of convection. Only for a certain range of Clapeyron slopes can the phase change induce a strong episodic thermal convection. ¿ American Geophysical Union 1994