Neutrally buoyant particles in low Reynolds number, pressure-driven suspension flows migrate from regions of high to low shear. When the particle density differs from the suspending fluid, buoyancy forces affect this particle migration. The ratio between the buoyancy and viscous forces, as quantified by a dimensionless buoyancy number, determines the phase distribution of the fully developed suspension. Electrical resistance tomography (ERT) was used to visualize and quantify particle migration in low Reynolds number pressure-driven pipe flows of dense and light particles suspended in a viscous fluid. The measured phase distributions are compared to the predictions of a modified suspension balance model.