Abstract The isothermal nonstationary diffusion of solute through a doublelayer membrane separating two external cells containing binary solutions is investigated. Expressions are derived which describe the concentration profile in the membrane as a function of time for situations of stirring, flushing or free diffusion in one or both external cells. These concentrations profiles lead to a description of the time dependency of several relevant quantities (e.g. the concentration difference over the membrane thus the osmotic pressure, the solute flux into each external cell, and the amount of solute remaining in the membrane). The decay of the osmotic pressure across a doublelayer membrane at zero volume flow is investigated theoretically and experimentally for the case of symmetrical external cells in which free diffusion takes place. It is shown that in this case a change of orientation of an asymmetrical membrane towards the external solutions does not lead to a different pressure—time curve. Practical equations are given allowing the determination of both the overall solute permeability and the average reflection coefficient of the membrane from a single pressure—time curve. Transient osmotic pressure across cellulose membranes separating solutions of glycerol trioctadecanoate or pentaerythrityl tetraoctadecanoate in toluene were measured with a high speed osmometer. Doublelayers were composted by placing two homogeneous membrane elements in series. The transport properties of the separate elements as well as of the combination could be measured. A concentration dependency of transport parameters is observed but appears to be of minor importance.