AbstractNonliving biomass of the common seaweed Ascophyllum nodosum is capable of accumulating cobalt from aqueous solutions to the extent of 160 mg Co2+/g. Successful desorption of cobalt from the biomass by acidic CaCl2 solutions revealed that the metal uptake phenomenon is reversible, implying physical sorption of cobalt. Chemical and instrumental analysis including electron microscopy, infrared (IR) spectroscopy, X‐ray dispersion and diffraction analysis provided supporting evidence that the biosorption mechanism involves predominantly ion exchange. Alginates of the cell wall (‐COOH groups) play an important role in cobalt binding. Coordination and sorption in the cell wall structure occur simultaneously and rapidly whereas penetration of cobalt into the cell occurs at a lower rate.