We have employed a three-dimensional compressible convection model to study the dynamics of phase transitions in the Martian mantle. A large core model with two exothermic phase transitions, the olivine to &bgr;-spinel and the &bgr;- to &ggr;-spinel transition, and a small core model including also the endothermic spinel to perovskite transition have been considered. The two exothermic transitions create 'thermal barriers' for small upwellings due to the latent heat consumption from the phase change. Upwelling plumes lose part or all of their buoyancy, which causes the formation of one stable area full of plumes. This tendency for the merging of plumes increases with internal heating. This type of convective planform is consistent with the relatively few large volcanic centers. The presence of a 175 km thick perovskite layer above the core-mantle boundary (CMB) yields a similar flow pattern, albeit with an even smaller number of plumes. However, the excess temperatures of the plumes and the mantle flow velocities in the lower mantle are smaller than those found in models without perovskite layer. The phase transitions cause an increase of temperature near the CMB, which prevents the lower mantle and the core from extensive cooling. A model with a perovskite layer decreasing in thickness with time can account for a peak in volcanic and magnetic activity early in the Martian history. ¿ 1998 American Geophysical Union