Different transport parameters (maximum retention, hydraulic and osmotic permeabilities) for a composite membrane with Na2SO4 and MgSO4 solutions, for concentrations ranging between 10(-3) N and 5 x 10(-2) N, were determined from reverse osmosis measurements. The influence of pressure on some characteristic parameters of the porous support, such as cation transport numbers and electrical resistance, were also studied. Differences in the values of these parameters are attributed to some changes in the structure of the porous layer due to pressure.