An apparatus is described which permits the determination of the mechanical permeability and the reflection coefficient of synthetic membranes at different hydrostatic mean pressures in the pressure range 1 up to 100 atm. Thus, it is possible to measure the volume flux across a membrane at pressures up to 100 atm using small pressure differences, &Dgr;P, across the membrane. Therefore, the linear relations of the thermodynamics of irreversible processes can be used for the evaluation of the transport properties at large mean pressures. Furthermore, it is also possible to perform measurements at large pressure and osmotic pressure differences where the usual linear relations of thermodynamics of irreversible processes may no longer apply. Some preliminary results obtained with asymmetric cellulose acetate membranes which were annealed at different temperatures are reported. These results show that the transport coefficients of the membranes depend on the mean pressure.