Although the time-lag method is one of the most common tools for the characterization of gas permeation membranes, it is entirely unknown in the vast area of liquid permeation membranes. The time-lag method requires a dynamic permeation test initiated by a step-change in the driving force for the species permeation, which is relatively easy for gas permeation, but not for liquid permeation. In this paper, we present preliminary trials to overcome this obstacle for liquid permeation membranes by performing a sudden change in the osmotic pressure by changing the draw solution concentration. To accomplish this, we have modified a standard forward osmosis testing system to monitor the changes in the mass of feed and draw solutions, and the conductivity of feed solution in real-time. To demonstrate the system's operation, we tested commercial, low-pressure reverse osmosis thin film composite membranes in the active-layer-facing-draw-solution orientation. We present and discuss the resulting time-dependent changes in the water flux and the reverse salt flux, and the processing of the experimental data. The shapes of time-dependent water and salt masses are similar to the pressure decay curves and pressure rise curves, respectively, in a time-lag gas permeation experiment. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.