In this research work, the biosorption of thorium from aqueous solutions by orange peel (OP) and orange peel cellulose (OPC) was studied and compared with each other. The kinetic studies showed that the Elovich and pseudo-second order kinetic models are well matched with the experimental results for the OP. For the case of OPC, the pseudo-second order kinetic models are well matched with the experment. The Langmuir, Freundlich, Dubinin–Radushkevich, Temkin and Redlich-Peterson isotherm models were also investigated. In order to find the best isotherm, three error analysis methods, i.e., correlation coefficient (R2), root mean square error (RMSE) and Chi square (χ2) were used. The error analysis showed that Redlich-Peterson, Temkin, Dubinin–Radushkevich and Freundlich models had miner errors, while the OPC Freundlich model had the smallest. Experimental results showed that the time required to reach the equilibrium is 4 h in the case of using OP as a biosorbent. In the case of using OPC as a biosorbent, the needed time to reach the equilibrium was about 1 h. Using OPC increases the adsorption rate and reduces the needed equilibrium time.