Previous studies of the effects of silicate phase transitions on mantle convection have tended to use numerical models in which the phase transitions have the same depths as found in the Earth's mantle, or in which a single transition is placed at a depth equal to half the layer thickness. This study presents a systematic investigation designed to examine the effects of an endothermic phase transition near the core-mantle boundary of terrestrial type planets. The results indicate that a deep mantle endothermic phase change may trap the lower thermal boundary layer, allowing only one upwelling to penetrate the phase transition. In these situations the deep endothermic phase change excites a dipolar convection pattern. This mechanism may have a role in the degree one power observed in the distribution of the Earth's hotspots and the development of the Martian crustal dichotomy. ¿ American Geophysical Union 1995