A dynamic model is developed for a tubular reverse osmosis unit. The proposed model considers the unit as a series of single tubes, each tube is described by a single ordinary differential equation (ODE) and the whole module is to be described by sets of differential algebraic equations. The tubes are modeled and solved sequentially where the output of any tube becomes the input for the next tube. The predictions of steady-state and dynamic models are in good agreement with the experimental results of a lab scale RO unit. The model equations are simpler and faster to solve than the currently used distributed parameter models. Furthermore, since it is based on first law principles, the proposed model offers better insight than input-output (black box) models into the effect of different design and operating parameters.