Radioactive wastes from human activities can spread to the environment and pose a serious threat to human health. This paper examined the removal of radioactive U(VI) and Th(IV) ions from aqueous solutions by a modified alga, Cystoseira barbata (Stackhouse) C. Agardh. The effects of pH, contact time, radioactive ion concentration, and temperature were examined on the adsorption and desorption of the radioactive ions by the algae from different solutions. The maximum adsorption capacity (q(ma)x) value was 217.39 mg g(-1) at 45 degrees C for U(VI) and 305.81 mg g(-1) at 45 degrees C for Th(IV). The data were analyzed with Langmuir and Freundlich isotherms. Scanning electron microscopy and Fourier transform infrared spectroscopy analyses were performed to determine the changes on the surfaces of the algae following adhesion of radioactive ions to the algal surfaces. The success rate for recovery of the loaded radioactive materials was more than 90%.