This study focuses upon the nature of the biosorption of nickel by a brown seaweed Ecklonia maxima and two seaweed-derived materials, dealginated seaweed waste and alginate fiber. An ion exchange relationship in the sequestration of nickel ions and the concomitant release of calcium ions has been identified. A millimolar equivalent relationship between nickel uptake and calcium release has been elucidated. Alginate fiber was found to adsorb nickel ions faster than either dealginate or E. maxima. This study is significant in that it sheds some more light on the chemical interactions that occur in biosorption processes. Full understanding of the mechanisms of biosorption and associated chemical interactions will allow the development of full-scale wastewater treatment systems.