A new type of redox sensor is developed on the basis of D,L-camphor sulfonic acid (CSA) doped polyaniline (PANI) membrane without Pt. CSA doped PANI membranes are redox active both at acidic and neutral pH. The results of potentiometric calibration demonstrate Nernstian response of transmembrane electrical potential for the redox couples of Fe2+/Fe3+ and Fe(CN)(6)(4-)/Fe(CN)(6)(3-). The transmembrane potential closely correlates the changes of redox potentials with Fe2+/Fe3+ and Fe(CN)(6)(4-)/Fe(CN)(6)(3-). The membrane also has a good response to those redox active substances that normally do not have satisfactory response on Pt electrode, such as L-ascorbic acid, Neutral red, Nile blue and N-phenylanthranilic acid. The lower detection limits for these substances are near 0.2 mM. In the absence of redox processes it is also possible to measure Cl- concentration at least from 0.005 mM to 0.1 M. It is demonstrated that the transmembrane potential across the doped PANI membrane is a mixed potential due to both electron transport in redox processes and Cl- ion transport. A new equation for transmembrane electrical potential is suggested and analyzed. (C) 2007 Elsevier B.V. All rights reserved.