A simple and highly sensitive electrochemical method for the simultaneous and quantitative detection of hydroquinone (HQ) and catechol (CT) was developed, based on a poly(thionine)-modified glassy carbon electrode (GCE). The modified electrode showed excellent electrocatalytic activity and reversibility towards the oxidation of both HQ and CT in 0.1 M phosphate buffer solution (PBS, pH 7.0). The peak-to-peak separations (Delta E-p) between oxidation and reduction waves in CV were decreased significantly from 262 and 204 mV at the bare GCE, to 63 and 56 mV, respectively for HQ and CT at the poly(thionine) modified GCE. Furthermore, the redox responses from the mixture of HQ and CT were easily resolved in both CV and DPV due to a difference in the catalytic activity of the modified GCE to each component. The peak potential separation of ca. 0.1 V was large enough for the simultaneous determination of HQ and CT electrochemically. The oxidation peak currents of HQ and CT were linear over the range from 1 to 120 mu M in the presence of 100 and 200 mu M of HQ and CT, respectively. The modified electrode showed very high sensitivity of 1.8 and 1.2 mu A mu M-1 cm(-2) for HQ and CT, respectively. The detection limits (S/N=3) for HQ and CT were 30 and 25 nM, respectively. The developed sensor was successfully examined for real sample analysis with tap water and revealed stable and reliable recovery data. (C) 2011 Elsevier Ltd. All rights reserved.