Abstract: An apparatus for determining data from which to calculate relative permeability of a porous body to a first fluid and a second fluid comprising a holder that renders all but first and second outer faces of the porous body impermeable to the first and second fluids. First and second capillary barriers respectively cover the first and second outer surfaces of the porous body. The barriers are permeable to the first fluid and impermeable to the second fluid. A first fluid flow system brings the first fluid to the outer surface of the first porous barrier, flows the first fluid under pressure through the first porous barrier, the porous body and the second porous barrier, and carries the first fluid away from the outer surface of the second capillary barrier. A second fluid flow system flows the second fluid under pressure through the porous body and carries the second fluid away from the second outer surface of the porous body.