We have performed experiments and simulations to study the accuracy of K r determination by the membrane technique. We have developed a numerical simulator and an analytical model which both account for the membranes as a boundary condition. The analytical model is based on the assumption of constant K r and linear P c during a small pressure step. These assumptions lead to a saturation equation which can be solved by standard methods used for thermal problems. The conclusions of the experiments and the simulations are the following: (1) for intermediate saturation values, the transient production is governed by membrane permeability, without any real influence of the K r , when the membrane resistance (thickness/permeability) is larger than core resistance; (2) for oil/water systems, the transient production is controlled by the very small mobility of the fluid at low saturation near the end-points. Its mobility can be determined with good accuracy, especially for very small K r values. But, the mobility of the other fluid (at the highest saturation) cannot be determined; (3) for oil/gas systems, the oil permeability can be determined, provided that the resistance ratio is smaller than 1 (membrane resistance/core resistance). Our conclusion is that transient production during P c measurement with porous plate or membrane techniques cannot be used for K r determination of liquid/liquid systems, except if the two following conditions are satisfied: (1) very low membrane resistance (smaller than core resistance) (2) independent measurement of the K r (max) at end-points by removing the membranes and using a standard one-phase flow.