The adsorption of diisooctyl sulfosuccinate (AOT) from high ionic strength solution onto well-defined calcium oxalate monohydrate (COM) and dihydrate (COD) crystals was studied. Adsorption at the COM/solution interface is characterized by a two-step (LS type) isotherm, starting at low equilibrium concentrations (5 mg dm(-3)). At c(eq) = 15-30 mg dm(-3) it reaches a plateau which is followed by a relatively steep inflection (c(eq) = 30-50 mg dm(-3)) and a further slow increase of the adsorption as a function of increasing AOT concentration. In suspensions without surfactant the particles were negatively charged. Upon adsorption an initial slight decrease of the negative zeta-potential, coinciding with the first plateau, occurred which was followed by a sharp increase in concordance with the increasing surface concentration of the surfactant. In contrast, adsorption onto COD is characterized by a sigmoid isotherm. It commenced at 14 mg dm(-3) of AOT and increased abruptly up to a plateau. The maximum adsorbed amount was about half the maximum amount adsorbed on COM, the corresponding adsorption densities in molecules per square nanometer being 7.45 for COD and 14.22 for COM, respectively. COD crystals suspended in electrolyte solution without surfactant were almost uncharged, and the negative zeta-potential increased in concordance with AOT adsorption. The results are discussed in accordance with literature data and by considering the ionic structure of the different crystal faces. We assume that the first adsorption step in the COM/surfactant system is due to electrostatic interactions causing head-on adsorption of the surfactant molecules at high-energy sites, while in the second step a bilayer is formed. In the COD/surfactant system the hydration layers covering the COD crystal faces are shielding them from electrostatic interactions. Consequently AOT adsorption at the COD/solution interface proceeds only through surface aggregation, resulting in a bilayer of intertwined surfactant molecules.