A new semi-empirical model for the pure water permeability in polymeric membranes for pressure-driven membrane separation processes is presented. The proposed new model is based on a combination of two well-known models, the solution diffusion (SD) and the pore flow (PF). The new model, the SDPF, assumes that water transfer across the membrane is jointly carried out by diffusion and pore flow mechanisms. This model may be useful to explain the effects of membrane micro-structural parameters and water physical properties on the permeability. This model further incorporates two empirical correlations for the operational conditions of the trans-membrane hydraulic pressure difference and the temperature. The SDPF model permeability function is to be regarded as the upper reference limit for the membrane permeability when using aqueous solutions as feed, as described in Part II of this work. (C) 2010 Elsevier B.V. All rights reserved.