A pore-scale experimental investigation of microscopic steady-state flow during co-injection from very low to high flow rates in the pore space of a sandstone is applied using 4D synchrotron X-ray micro-tomography to advance our understanding of flow regimes. We report the results of micro-CT imaging experiments directly visualizing the simultaneous flow of both a wetting and a non-wetting fluid through a Bentheimer sandstone, at pore-scale resolution. For small flow rates, both fluids flow through unchanging, distinct, bicontinuous 3D pathways. At higher flow rates, however, the non-wetting fluid continually breaks up into discrete ganglia; these are then advected through the medium. We propose that the non-wetting fluid breaks up when the sum of the viscous forces exerted by the wetting and the non-wetting fluids exceed the capillary forces at the pore scale.