A new thermodynamic model is developed for water and solute transports through reverse osmosis membranes. The model is featured with rigorous derivations in theoretical development and clearly defined parameters for membrane transport properties. The new model can correctly describe not only the dependence of salt rejection on pressure and salt concentration, but also the non-linearity between water flux and pressure. Comparisons of model simulations with the reported reverse osmosis experiments demonstrate that the parameters in the new model are concentration-independent. This study shows that water and salt transports through reverse osmosis membranes can be satisfactorily described with irreversible thermodynamics.