A modified electrode sensor for the detection of thiols is described. The sensor was constructed by incorporation of the coenzyme pyrroloquinoline quinone (PQQ) into a polypyrrole (PPy) film on a glassy carbon electrode substrate by the electropolymerization of pyrrole in the presence of PQQ. The electrochemical properties of entrapped PQQ in the PPy film were influenced by the applied potential during electropolymerization and by film thickness, both of which were optimized to yield a stable and reproducible response for entrapped PQQ. The PQQ/ PPy sensor was utilized for the amperometric detection of cysteine, homocysteine, penicillamine, N-acetylcysteine, and glutathione. The response for each thiol in pH 8.42 borate buffer was found to be linear with detection limits (S/N = 3) ranging from 13.2 microM for glutathione to 63.7 nM for cysteine with sensitivities of 0.023 nA/microM and 4.71 nA/microM, respectively. The response and detection limits were found to be sensitive to the nature of the thiol and the solution pH. Furthermore, in the presence of dopamine, ascorbic acid, or uric acid, the pH-dependent redox potential of the PQQ catalyst allows tuning of the detection potential to enhance the selectivity for thiols over these potential electroactive interferences.