We measure the three-phase oil relative permeability kro by conducting unsteady-state drainage experiments in a 0.8m water-wet sandpack. We find that when starting from capillary-trapped oil, kro shows a strong dependence on both the flow of water and the water saturation and a weak dependence on oil saturation, contrary to most models. The observed flow coupling between water and oil is stronger in three-phase flow than two-phase flow, and cannot be observed in steadystate measurements. The results suggest that the oil is transported through moving gas/oil/water interfaces (form drag) or momentum transport across stationary interfaces (friction drag). We present a simple model of friction drag which compares favorably to the experimental data.When the single-phase Darcy equation is generalized to multi-phase flow in porous media, it is assumed that each phase flows due to the pressure gradient within that phase, albeit with a reduced permeability [1,2]. Conceptually, each phase flows in a capillary-stable reduced network compared to single phase flow. The change in permeability is parameterized by the relative permeability, k ri , that is assumed to be a function of the saturation of the phase, S i (the local volume fraction of the pore space filled by the phase i). Mathematically this is expressed through the Darcy-Buckingham equation [1].Here, q i , µ i , ρ i and P i are flux, viscosity, density and pressure of phase i flowing through porous media of permeability k. While this multi-phase flow equation is widely used due to its simplicity, it is known to break down at high capillary numbers (the network is fluid) [3], unstable flow [4], high viscosity ratio (due to viscous coupling between the mobile phases) [5], and three-phase flow (the network for the intermediate phase depends on the other two phases) [6].Here we concentrate on the combined effects of threephase flow and viscous coupling. Three-phase flow occurs when three mobile fluid phases co-exist in a porous media; typically water is the most wetting phase and oil the intermediate wetting phase. Three-phase relative permeability has been measured using steady state experiments [6], and many empirical models of the oil relative permeability k ro have been introduced [7]. These models have a dependence on both saturations, k ro (S o , S w ), based on the idea that the connected oil network depends on the amount of water. From observations in micromodels [8,9] and capillary stability arguments based on geometry [10] and thermodynamics [11] various three-phase pore level fluid configurations and flow mechanisms have been recognized. These mechanisms have been incorporated into network models to predict three-phase relative permeability and saturations path [12][13][14], under the ansatz of each phase flowing independently in its own network. * dicarlo@mail.utexas.edu Viscous coupling, where the pressure gradient of a particular phase affects the flow of the other phase, has been investigated for two-phase flow through experiments [15][16][17], analytical...