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End point relative permeabilities were measured in three limestones with permeabilities ranging from 0.6 to 220 mD under five wettability states established by adding different organic acids, of similar molecular structure but different alkyl chain length, to the oil phase. The altered wettability corresponding to each oil/brine pair is characterized by their dynamic contact angle on a polished calcite substrate, 0(w), which varied between 50 degrees and 150 degrees. Saturation-normalized relative permeability to oil exceeds one at 0(w)< 140 degrees in all rock considered. The equivalent slip length, defined by modeling the porous medium as a capillary tube with the defending phase distributed as an annular film on the tube wall, was below 200 nm in all experiments. The results indicate that commonly used models of relative permeability, which assume that the maximum permeability is the single-phase permeability, underestimate oil displacement for a much wider range of contact angles than previously documented.