The biosorption potential of chemically modified onion skin (CMOS) for cadmium (II) ions (Cd2+) from its aqueous solutions at different conditions of initial Cd2+ concentration, contact time, pH, and temperature was investigated under batch mode. The results showed that biosorption of Cd2+ is influenced by these different conditions. The equilibrium biosorption data were analyzed by two-parameter (Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R)), three-parameter (Redlich-Peterson, Sips, Toth, and Khan), and four-parameter (Baudu and Fritz-Schluender) isotherm models using linear and nonlinear regression methods. The best-fitting isotherms to describe the biosorption data of the Cd2+ CMOS system were found to be those of Freundlich, Sips, and Fritz-Schluender for the two-, three-, and four-parameter models, respectively. A maximum biosorption capacity (Q(max)) of 18.34 mg/g was obtained for CMOS as compared with 11.90 mg/g for raw onion skin. The biosorption kinetics followed a pseudo second-order kinetic model and the biosorption mechanisms were controlled by boundary layer surface diffusion. The thermodynamic parameters of Gibbs free energy (Delta G(o)), enthalpy (Delta H-o), entropy change of biosorption (Delta S-o), and activation energy (E-a) were evaluated and it was found that the biosorption process was spontaneous, feasible, endothermic, and predominantly a physisorption process, however, with some element of chemisorption. A single-stage batch adsorber was designed for different biosorbent dose-to-effluent volume ratios using the Fritz-Schluender equation. The CMOS could be regenerated and reused for up to five cycles with no significant loss of efficiency. Therefore, CMOS has potential for application as an effective biosorbent for the removal of Cd2+ from aqueous solutions.