The effects of pH and supporting electrolyte concentration on the passive transport of an ionized (cationic or anionic) drug through a thick fixed charge membrane have been theoretically studied. This system constitutes a simplified model for the pH controlled ion transport and drug delivery through membranes of biological and pharmaceutical interest. Calculations were carried out for different values of the membrane fixed charge, supporting electrolyte and drug concentrations covering a broad range of the conditions usually found in experiments. The theoretical approach employed is based on the Nernst-Planck flux equations, and all of the species present in the system (the neutral and ionized forms of the drug, the two supporting electrolyte ions and the hydrogen and hydroxide ions) have been taken into account without any additional assumption. It has been shown that the Goldman constant field assumption together with the total co-ion exclusion assumption provide good approximated solutions for high membrane fixed charge concentrations. The model predictions show that the internal pH within the membrane, the total drug flux and the membrane potential are very sensitive to the external pH values. Comparison of our results with available experimental data confirms the potential utility of the calculations for the analysis and design of experiments involving the pH dependent passive transport of ionized drugs through a fixed charge membrane, especially in the cases of thick biomembranes, biochemical sensors and pH-controlled drug delivery systems.