This study compares different 3D spherical models of thermal convection in the Martian mantle. The best fit to the Tharsis bulge is obtained if the endothermic phase transition to perovskite occurs closely above the core-mantle boundary. In this case, a large-scale flow develops and the associated geoid anomaly explains approximately half of the observed long-wavelength geoid. The shallower exothermic phase transitions have little influence on the planform of the flow and on the shape of the geoid. Previous studies that included exothermic transitions were biased by effects of a soft lithosphere, which introduces a large-scale mantle flow even without a deep endothermic phase transition. However, such models predict a continous resurfacing and a dominant geoid low, which is in conflict with observations. ¿ 2000 American Geophysical Union