Literature models for reverse osmosis are based either on irreversible thermodynamics (IT) or on transport mechanisms. Our research focuses on the evaluation of existing models through comparison of experimental and predicted data for the transport of solute and solvent through the membrane. The present paper reviews the fundamentals and design equations of the IT theory. Our experimental and literature data are thereafter used to check the validity of the IT approach. A further paper will similarly evaluate the transport models. Experimental data confirm that the osmotic pressure can be predicted with great confidence by the Fitter equation for 2-1 and 2-2 electrolytes over a wide range of concentrations. Moreover, divalent ions and/or 2-2 electrolytes are rejected to a greater extent than, respectively, monovalent and/or 2-1 electrolytes. The results of the model analysis confirm the validity of the Kedem-Katchalsky predictions at high values of the volume flux and of the overall validity of the Spiegler-Kedem approach. The transformed results, however, illustrate that both cross coefficients are very small hence making it impossible to pass judgement on the applicability or otherwise of the Onsager reciprocal relations.