It is demonstrated theoretically that the diffusion flow of solute from concentrated solutions through composite/asymmetric (reverse osmosis) membranes can be considerably reduced due to the concomitant osmosis. Therefore, the diffusion permeability of such membranes is strongly dependent on the hydraulic conditions (zero trans-membrane hydrostatic pressure difference vs. zero trans-membrane volume flow). Besides that, under osmosis conditions, the trans-membrane diffusion flow of solute is demonstrated to be a strongly non-linear function of concentration in the source solution despite the fact that the diffusion permeabilities of constituent layers are assumed independent of solute concentration. The diffusion flow is also shown to noticeably depend on the membrane orientation with respect to the concentration gradient. (C) 2010 Elsevier B.V. All rights reserved.