The liquid permeability of composites of fiber and superabsorbent polymer (SAP) is critical to the proper performance of absorbent hygiene products such as infant diapers. Previous work has focused on empirically improving performance of absorbent composites by modifying the components, but a systematic study of the parameters affecting permeability of these systems has not been published. A good model of permeability would aid our understanding of the complex phenomenon of liquid permeability of pads that change in volume and porosity during swelling. In the present work, a mathematical model of liquid permeability in both gel beds and composites of fiber and gel is developed. The model uses a Carman-Kozeny approach with added calculation of the porosity and specific surface area that result upon swelling of the SAP, as well as subsequent compression of the swollen composite. The model incorporates ten parameters that are necessary to determine the permeability of the system. The implications of the model for improving permeability in absorbent composites are explored. (c) 2006 Wiley Periodicals, Inc.