Considerable attention has been focused on the reduction of azo dyes discharged from dyeing, textile and other industries since some of them or their metabolites may cause toxicity. The efficient treatment of these effluents at industrual scale presents many difficulties, particularly at high dye concentrations and at low energy consumptions. Anaerobic biodecolourisation seems to be the most economic and environmentally friendly method for azo dye wastewater treatment. In a recent study of the authors1, a novel-type bioreactor with activated carbon (AC) was developed providing high azo dye degradation rates at very short space times/hydraulic residence times. Additionally, a model was proposed involving both heterogeneous catalysis and biological decolourisation. In catalysis, activated carbons have been mainly used as support, but their use as catalysts on their own is growing quickly. One of the advantages of ACs is the possibility of tailoring their physical and/or chemical properties in order to optimise their performance for specific applications.2 Therefore, it is important to analyse the effect of texture and surface chemistry of AC in azo dye degradation by using ACs with diverse average pore sizes and/or surface chemistries in the bioreactors. This work deals with texture-modified activated carbons for the decolourisation of azo dyes in upflow stirred packed-bed reactors (USPBRs) containing a biological activated carbon system. The preparation, modification and characterisation of pore size-modified activated carbons have been completed. The initial material selected was a commercial activated carbon (Norit Rox 0.8). ACs with larger porosities were obtained by CO2 gasification of the raw carbon previously impregnated with cobalt. The role of cobalt was to catalyse the gasification of carbon, thereby promoting the formation of mesopores. The gasification experiments were carried out in a tubular vertical reactor. ACs with different average pore sizes were produced by applying different gasification times. The textural characterisation was based on N2 adsorption isotherms at 77 K. The performance of these carbons on azo dye biodegradation in the USPBRs is being evaluated.