The AC impedance characteristics of planar solid-state electrochemical carbon monoxide (CO) sensors with Nafion as the solid polymer electrolyte are reported. For a re-cast Nafion(R) sensor design, Nyquist plots indicated very high impedances (10(7)-10(8) Omega), which slightly decreased with an increase in the working electrode surface area (0.25-0.50 cm 2). Only one enlarged semi-circle was observed, relating to an equivalent circuit with a geometrical capacitance (C-g) in parallel with a bulk resistance (R-b). In contrast, a sensor design using a commercial membrane produced a system with a much lower impedance (< 10(3) Omega), with Nyquist plots showing a flattened semi-circle at high frequencies (due to C-g and Rb in parallel), followed by the start of a large semi-circle (due to the double-layer capacitance, C-dl, in parallel with the charge-transfer resistance, R-ct) over a lower frequency range. On addition of CO, the second circular arc reduced in size, indicating that the interfacial impedance of the electrochemical system had decreased.