Wastewater containing mainly 5-amino-6-methyl-2-benzimidazolone (AMBI), used in the manufacture of dyes, was characterized as bio-recalcitrant by means of different biodegradability tests. In order to enhance the biodegradability of this important pollutant, solar photocatalytic degradation methods were explored. The systems light/TiO2/O-2, light/TiO2/H2O2, light/Fe3+/O-2 and light/Fe3+/H2O2 were compared under direct sunlight at the Plataforma Solar de Almeria (Spain), using a Compound Parabolic Collector (CPC). The iron photo-assisted systems exhibited the most interesting behaviour, from the kinetic and engineering points of view, especially if their combination (as pre-treatment) with a biological process is considered. To compare the efficiency of these systems, the evolution of the following parameters were studied: (a) the dissolved organic carbon and initial compound concentration, (b) the toxicity, and (c) the biodegradability of treated solution. At lab scale, using a solar lamp, the degradation rate of the system light/Fe3+/H2O2 was two times higher than the system light/Fe3+/O-2 but this last system does not need H2O2 addition, improving the economical requirements of the system. (C) 2003 Elsevier Ltd. All rights reserved.