The sorption of Cu(II) by chitin, a naturally occurring material, and chitosan, prepared from chitin, was investigated and compared. The effect of pH, initial metal ion concentration, particle size, sorbent concentration, and stirring rate on sorption capacity was characterized by measuring the sorption isotherms. Sorption data of chitin and chitosan were best modeled by the Langmuir and Redlich-Peterson isotherms, although they can be modeled by the Freundlich and BET sorption models. Next, batch studies were carried out to identify the rate-controlling steps for Cu(II) sorption. Single resistance models were used in the determination of the external film mass transfer step and the intraparticle diffusion step.