This work reports the electrochemical detection of catechol (10(-4) to 10(-6) M) in the presence of ascorbic acid (10(-4) to 10(-6) M) using dual-band poly(3-methylthiophene) (P3MT) electrode flow-injection amperometry without prior separation. The selectivity involved in this method is based on the differences in electrochemical behavior of catechol and ascorbic acid. A variety of dual-band Pt, Au, glassy carbon, and P3MT electrodes were constructed and used as the electrochemical detector. The upstream electrode of the series dual-band electrode unit is used for detection of catechol and ascorbic acid and the downstream electrode for detection of the oxidized catechol. The band dimensions range from 0.1 x 2.5 to 1 x 5.5 mm with the interelectrode gaps varying from 0.1 to 0.5 mm. Although this method is effective for dilute solutions of catechol (<10(-5) M) in the presence of ascorbic acid (<10(-5) M) under both neutral and acidic conditions, it is not efficient for more concentrated mixtures (>10(-5) M) unless the measurements are made under acidic conditions such as at pH 1.6. A negative deviation from the ideal calibration curve of the oxidized catechol reduction is found in the more concentrated mixtures (>10(-5) M) at physiological pH 7.4. Charge-dipole interaction and hydrogen bonding between the oxidized products of,catechol and ascorbic acid in the concentrated neutral solutions are proposed to explain the suppression of the current signals. Variation of the flow rates from 0.5 to 3.0 ml/min has no effect on the performance of the detector. The dual-band P3MT electrode has less positive oxidation potentials for catechol and ascorbic acid oxidation compared to the bare Pt, Au, and glassy carbon electrodes because of the catalytic activity of the P3MT electrode surface. Dual-band P3MT as well as glassy carbon electrodes have the best (highest) collection efficiency for catechol detection. The collection efficiency also remains constant when the interelectrode gaps vary from 0.1 to 0.5 mm.