The pH-switchable ion transport and selectivity in nanopore membranes with fixed charges is theoretically analyzed using a model based on the Nernst-Planck flux equations. The ionization state of the weak polyelectrolyte groups attached to the pore surface appears to be a crucial factor in order to understand the conductive properties of. the pore. The model allows for the calculation of the fluxes of all of the ionic species involved and the membrane potential. Comparison of the theoretical results with the experimental data by Martin el al. (Science 1995, 268, 700-2; J. Phys. Chem. B 2001, 105, 1925-34; and Adv. Mater. 2001, 13, 1351-62) and Stroeve et al. (Langmuir 2000, 16, 2401-4; and Langmuir 2001, 17, 5271-5) for surface modified nanopores with fixed charges shows that the model can describe qualitatively the changes of the permeate ion flux and the membrane potential with the pH and the solute concentration of the external solutions. The effect of applying an electric potential to the nanopore walls is also studied. Some of the main characteristics that allow a simple description of pH-switchable effects on nanopore membranes with fixed charges are clearly shown.