The performance of unmodified and cetyldimethylethyl ammonium bromide (CDAB) modified nonviable Aspergillus oryzae for removal of Acid Blue 25 (AB 25) and Acid Red 337 (AR 337) was investigated in single and binary systems. In single system, the biosorption capacities of CDAB-modified biosorbent reached 160.36 and 280.39 mg g(-1) for AB 25 and AR 337, respectively, which were 1.52 and 1.66 times higher than that of unmodified biosorbent. In binary system, the biosorption capacities of unmodified and CDAB-modified biosorbents for both dyes decreased significantly compared to that in single system. Relative competitiveness analysis demonstrated that there existed critical initial concentration ratio which determined the predominance of dyes during biosorption process. The biosorption of AB 25 was found to be in dominant position at initial concentration ratio of [AB 25]/[AR 337] above 0.63. Kinetic analysis indicated that intraparticle diffusion was the limiting step for biosorption of two dyes onto biosorbents. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.