The biosorption of five metal ions, cadmium, copper, nickel, lead and zinc in single component solutions and a multi-component mixture in aqueous solutions by spruce sawdust is reported. A comparative study, performed through batch experiments, between the cationic release of sawdust in ultra pure water and in a metallic solution leads to an understanding of the heavy metal ion fixation mechanism on sawdust. An empirical model is developed in order to distinguish between a sorption phenomenon purely based on ion exchange from one in which physical adsorption is involved. Thus, the difference in cations released between the two solutions corresponds to the metal ions which bind to anionic sites by displacing protons from acidic groups and existing cations (natural components of the sawdust) from anionic sites. Metal ion binding capacities of spruce sawdust reach values ranging from 2.1 to 4.3×10⁻² meq g⁻¹ in single metal ion solutions with the following relative preference: Zn < Ni < Cd << Cu < Pb. Our results show variable contributions of the different cations of the sawdust in the ion exchange mechanism in which calcium, naturally present in the sawdust, accounts for about 65% of the biosorption of the metal ions studied. The study also indicates the possibility of desorbing the loaded metal ions with a calcium solution.