The degradation of the azo-dye acid orange 7 (AO7) was performed in a photoelectrochemical reactor module consisting of up to nine photoelectrochemical cells equipped with TiO2 electrodes deposited on transparent conducting substrates. These electrodes were illuminated from the backside by UVA broadband light or sunlight. Electrical bias was applied for efficient separation of photogenerated charge carriers. Solutions of AO7 (<= 1 mM) in dilute supporting electrolyte (10 mM HClO4 or Na2SO4) were recirculated through the reactor(s) at a fixed flow rate of 12.21 h(-1) (mean flow velocity of 4.24 cm s(-1)). AO7 disappeared following first-order kinetics. During solar irradiation, using a total electrode area of 576 cm(2), and 1.5 V bias with respect to a stainless steel counter electrode, a decay rate constant normalized to unit volume, k', of 0.23 cm(3) s(-1) was found under a photocurrent of 0.1 A. Concentrations could be decreased to the micromolar range within 1 h. (C) 2009 Elsevier B.V. All rights reserved.