This paper contributes to the comprehension of kinetic and equilibrium phenomena governing trace metal ion sorption on organic matter coated mineral particles. Sorption and desorption experiments were carried out with trivalent metal ions M(III) (M = Eu, Cm) and poly(acrylic acid) (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO4. Under these conditions, M(III) interaction with the solid is governed by sorbed PAA. The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Within less than 30 s, a state of local equilibrium is reached between M(III) and adsorbed poly(acrylic acid). M(III) bound to the organic-mineral surface and to dissolved PAA keeps five water molecules in its first hydration sphere. Interaction of M(III) with alumina-bound PAA appears to be stronger than with dissolved PAA. With increasing contact time, a change of the metal ion speciation occurs at the organic-mineral surface. This change is explained quantitatively by kinetically controlled reactions, which succeed a rapid local equilibrium. The experimental findings suggest, in agreement with model calculations, that a part of the initially sorbed M(III) is slowly converted to a kinetically stabilized species, thereby losing water molecules from the first coordination sphere as indicated by TRLFS. This species might be assigned as a ternary Al2O3-M(III)-PAA complex. The second part of the initially bound M(III) appears to experience as well kinetically controlled reactions, however, without showing changes in the first coordination sphere. We assume that the kinetic stabilization is the consequence of rearrangement processes of the PAA at the alumina surface.