The ternary system composed of metal ions (M), humic substances (HS), and mineral phases is actually not understood. Among the various equilibria considered, the most difficult to understand is the interaction of M with the mineral-bound HS. It appears actually established that an additive model based on data deduced from binary systems is not applicable. Two main reasons are suggested to explain this: the first one invokes the formation of a ternary surface complex where the metal ion is coordinated to both mineral and organic matter functional groups. The second reason raises the problem of using an electrostatic term derived from a model developed for pure mineral oxides for a surface coated by an organic layer. All studies generally consider that sorbed HS and soluble HS behave similarly with respect to M (same complexed species, same mechanistic interaction equilibria). The goal of the work is to check this point using a model ternary system composed of g-alumina, trivalent metal ions M(III) (M=Eu and Cm) and polycarboxylic acid (PAA). PAA is adsorbed on Al2O3 prior to M adsorption to study selectively M / sorbed PAA (PAAads) interaction. In order to neglect Eu-Al2O3 interaction, the work is performed under conditions where M interaction is dominated by the organic matter (pH=5, maximum organic loading). The characterization of the interaction mechanisms between M and free and sorbed PAA is based on sorption/desorption data as well as spectroscopic measurements (TRLFS, EXAFS). The results clearly show a difference for the M sorption on PAA and PAAads since M speciation and mechanisms appear different in both systems. The origin of this discrepancy may be explained by the difference in PAA “structure” in the two systems. When PAA is free in solution, in the range of PAA concentrations studied, the polymers are independent linear chains. When PAA is adsorbed on alumina, the polymer chains cannot any more be treated as independent chains. One may rather assume formation of aggregates that form an organic layer at the mineral surface which can be viewed as a penetrable, gel-like structure.