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.