The adsorption of humic and fulvic acids onto nitrate, chloride and carbonate intercalated layered double hydroxides with Mg/Al ratios ranging from 85/15 to 60/40 was studied by adsorption isotherms at different ionic strengths and the characterization of the preferentially adsorbed humic substance size fractions. The adsorption of humic and fulvic acids onto LDHs occurred by ion exchange with both the intercalated and surface anions of the LDH and ligand exchange reactions with surface groups. The contribution of both mechanisms to the total HA and FA adsorption was estimated. Lower molecular weight humic and fulvic acids were preferentially adsorbed because these fractions can more easily enter the mesoporous LDHs and contain more carboxylic groups, which are known to be involved in ligand exchange reactions with e.g. surface Al-OH groups. Intercalation of the entire HA or FA molecules in-between the LDH sheets is unlikely to occur. (C) 2007 Elsevier B.V. All rights reserved.