Abstract: A porous inorganic composite semipermeable membrane provided with a support and a separating layer which has pores with an average diameter less than 1 .mu.m and which comprises at least two refractory metal oxides, wherein:the separating layer is composed of grains of a first oxide phase and grains of at least one second oxide which is dispersed over and chemically bonded to the first oxide phase, so as to constitute at least a portion of the free surface of the grains of the first oxide phase and at least a portion of the grain boundaries between the grains of the first oxide phase;the first oxide phase and the second oxide are chosen so that the second oxide slows down diffusion both at the surface and at the grain boundary of the first oxide phase and has low solubility in the first oxide phase; andthe volume of the second oxide lies in the range of 0.1% to 25% of the volume of the first oxide phase.

Abstract: A process is provided for modifying a ceramic ultrafiltration membrane system consisting of a sub-microporous top layer and a microporous inorganic carrier whereby said top layer after modification has reduced pore size and/or is made catalytically active.