Reactive Black 5 was efficiently sorbed onto chitosan in acidic solutions through electrostatic attraction of anionic dye molecules to protonated amine groups of the polymer. Sorption capacities as high as 750-1000 mg of dye g(-1) were obtained. The addition of chitosan to dye solution strongly affected the pH of the solution and the sorption efficiency. To avoid this variation of the pH, chitosan was conditioned in sulfuric acid solutions to prepare protonated chitosan. This preprotonation induced a significant lessening of sorption properties. This impact depended on dye concentration, sorbent dosage, and sorbent particle size. The ability of dyes to form aggregates in solution (depending on pH, dye concentration, and ionic strength) might partly explain the impact of the preprotonation on uptake performance. Indeed, the agglomeration of dye molecules affects the ionic size of the solute and consequently its diffusivity in the polymer network and accessibility to internal sites. Particle size, sorbent dosage, and dye concentration significantly affect sorption isotherms and kinetics.