Geophysical and geochemical properties of partially molten regions of the Earth’s upper mantle are strongly affected by the distribution of melt. I investigated the mechanisms affecting the 3-dimensional (3D) melt distribution of partially molten harzburgite samples. The 3D melt distributions of experimental charges of various melt fractions were recorded using synchrotron-based X-ray microtomography. In most samples the melt fraction increases along the axial direction of the sample, but the mineral phases exhibit no systematic heterogeneities. Analysis of time-series samples confirmed that the melt fraction heterogeneity was generated during sintering and took more than 84 hours to develop. To elucidate the mechanisms of melt focusing, I evaluated the importance of gravity, surface tension, lithologic partitioning and thermal migration as potential driving forces. The scale of the melt fraction variations and the speed at which they develop appear compatible with the thermal migration model.