A　high　quality　copolymer　(CATCS)　based　on　starch　and　chitosan　was　synthesized　and　applied　for　the　enhanced　decolorization　of　aqueous　dye　solutions.　The　flocculant　was　characterized　by　scanning　electron　microscopy　and　Fourier　transform　infrared　spectroscopy.　The　effects　of　the　initial　pH,　dosage　of　CATCS,　initial　dye　concentration,　salt　concentration　and　reaction　time　on　the　decolorization　ability　of　CATCS　were　studied.　An　ultrahigh　decolorization　efficiency　(97.1-99.7%)　for　the　dye　solutions　was　observed　for　CATCS.　It　was　effective　in　a　wide　pH　range　(2　to　10)　and　could　withstand　a　high　salt　concentration.　The　CATCS　flocculant　was　ten　times　more　efficient　than　commercial　activated　carbon　in　the　treatment　of　real　dye　wastewater.　A　decolorization　efficiency　of　98.3%　for　the　real　dye　wastewater　was　reached　using　0.34　g　L-1　CATCS.　The　thermodynamics　and　kinetics　of　the　decolorization　process　were　investigated　in　detail　and　the　decolorization　mechanism　with　CATCS　was　determined.　The　enthalpy　change　was　over　41　kJ　mol-1　for　the　decolorization　process　of　dye　solutions　by　CATCS.　The　positive　entropy　change　value　for　the　decolorization　corresponded　to　an　increase　in　the　degree　of　freedom　of　the　adsorbed　species.　The　decolorization　process　was　endothermic　and　the　interaction　was　controlled　with　chemisorption.　©　The　Royal　Society　of　Chemistry　2015.