Membranes which give high fluxes without selectivities provide a large interfacial area between fluid phases which is independent of the normal constraints of loading and flooding. These membranes offer a largely unexplored route to separation process intensification. Three examples are given in the paper. First, studies of hollow fiber blood oxygenators show that flows which are only a few percent from uniform can reduce overall mass transfer by an order of magnitude. Second, flat sheet membrane evaporators promise the performance of a centrifugal evaporator at a fraction of that unit's cost. Third, distillation in hollow fibers means that the height of the transfer units can be smaller than 0.1 cm, indeed as small as desired. (C) 2003 Society of Chemical Industry.