The biosorption of Acid Red 14 (AR14) and Reactive Red 15 (RR15) by acid treated okara (ATO) in binary solutions was investigated. To overcome the severe spectral overlapping, partial least squares regression as a multivariate calibration method was successfully developed for the simultaneous determination of AR14 and RR15 in binary solutions. In single solution, the Langmuir-Freundlich isotherm yielded the best fit for the whole concentration range; however, both Langmuir and Freundlich isotherm predicted the experimental data well at the different concentration region (100-300 mg/L and 300-500 mg/L). Maximum biosorption capacity obtained from Langmuir isotherm for ARM and RR15 in single solution was 217.39 and 243.90 mg/g, respectively. In binary solutions, antagonistic biosorption occurred and ATO favored to adsorb RR15. Partial competitive Langmuir, Langmuir-Freundlich isotherm for a binary system and extended Freundlich isotherm could all represent the binary equilibrium data well. The FTIR results indicated the main functional groups on ATO for the biosorption would be amine, hydroxyl, carboxyl and either. ATO was proved to be a low-cost and efficient biosorbent for the removal of dyes studied. (C) 2011 Elsevier B.V. All rights reserved.