The flow in regions of oil and gas reservoirs that are dominated by gravity segregation can occur through two different mechanisms. First, all reservoir fluid phases, possibly oil, gas and water, might move through the same pores. One or more phases might move upward while the others move countercurrently downward, in the same pores. If such a mechanism were valid, Darcy's law could be applied uniformly over large areas of the reservoir to provide average-phase velocities, just as if the flows were the result of pressure-induced convection. However, a second mechanism is also possible. The phases might be segregated such that some pores carry the upward flowing phases, while other pores carry downward flows. When this mechanism is valid, Darcy's law applies only locally to the upward or downward regions of flow. The two mechanisms result in substantially different phase velocities. Simulators, including those available commercially, generally assume the first mechanism (i.e., uniform, countercurrent flow throughout each simulation cell). This paper describes an experimental study in which two-phase flow velocities were measured as the phases moved through a bed of small, uniform glass beads, as the result of gravitational forces only. The resulting velocities were more consistent with those predicted for segregated flows of Mechanism 2 compared with those of the commonly assumed homogeneous flows of Mechanism 1. These results suggest that greater accuracy in reservoir simulation may be achieved by including segregated flow in areas of the reservoir where gravity segregation predominates.