To fully evaluate the potential of RO and facilitate scale-up procedures, the modelling of the process is an important tool. The concentrate-side of the membrane (which is also the feed side) plays the hey-role. The determination of the flow regime and hydrodynamics in the concentrate-channel can be carried out through the measurements of the residence time distribution (RTD), which is used in our research. Results obtained stress its applicabilitly to this specific geometrical layout and demonstrate that the experimental average residence time of the concentrate channel is smaller than the theoretically calculated residence time, with differences between both values gradually decreasing with increasing liquid flow rate. This observation points toward the presence of dead volumes, which reduce the veal average residence time. The flow regime can be defined in terms of several model approaches. Application of a laminar flow pattern with a parabolic velocity gradient gives a reasonable fit of experimental RTD-curves. Experimental RTD curves can also be fitted using the Plug Flow with Dispersion model. Model parameters are determined from comparison of calculated and experimental RTD curves. The definition of this flow profile in the concentrate channel is important in the use of transport models to characterise the membrane performance, since mass transfer and pressure drop are a function of the flow regime.