Flow in porous media described by Darcy’s law extended to two-phase flow using the concept of relative permeabilities k _r naturally assumes a maximum value of 0 ≤ k _r  ≤ 1. Reports in literature and our own experimental data show endpoint relative permeabilities k _r > 1. In the porous medium, the flux of the non-wetting phase is in many cases about 2-4 times higher when a small amount of the wetting phase is present. Here, we draw an analogy between k _r > 1 and a slip-boundary condition for the pore scale flow. We use a model description assuming flow in capillary tubes with a slip boundary condition. This model predicts that the flux increase due to slip depends on the equivalent capillary radius of the flow channels. Our k _r data specifically follows this dependence indicating that slip is a plausible explanation for the observation of k _r > 1.