BACKGROUND: This study investigated steam-activation as a technology for adding value to fast-pyrolysis bio-chars. Due to short residence times in the fluidized-bed pyrolyzer, required to maximize bio-oil production, the bio-char structure tends to be underdeveloped thereby impacting its full potential application as a value-added co-product. RESULTS: The bio-chars of several substrates from a fluidized-bed fast pyrolyzer and their respective corresponding steam-activated counterparts were characterized for their surface areas and metal ion adsorption (copper, cadmium, nickel and zinc). Surface areas increased with activation from negligible to 136-793 m(2) g(-1) of material, with concomitant pore development evidenced by scanning electron microscopy and mathematical modeling. Affinity to Cu2+ was highest with adsorption efficiencies for 1 mmol L-1 solutions ranging from 60-85%. Metal ion adsorption performance was feedstock dependent and increased with activation, possibly due to improved access to highly reactive adsorption sites associated with the inorganic material in the feedstock. CONCLUSION: Because of their higher yields and metal ion uptake, broiler litter and alfalfa stems could be the feedstock of choice when considering upgrading fast pyrolysis bio-char for metal ion uptake. However, if the development of large surface areas is required, guayule bagasse and soybean straw could be the preferred feedstock. Published 2010 by John Wiley and Sons, Ltd.