The semi-empirical Pitzer equations provide the basis for the geochemical model that is used to describe the solubility and speciation of actinide ions (primarily Pu and Am) in the high ionic strength solutions that are native to the stratigraphic horizon of the Waste Isolation Pilot Plant. The Pitzer equations provide the single-ion activity coefficients for the An(III,IV,V) ions as well as inorganic (sulfate, carbonate, bicarbonate) and organic ligands (acetate, oxalate, citrate, EDTA) and actinide complexes. Solubility and speciation calculations were performed at Sandia National Laboratories using the in-house code, Fractured Matrix Transport, (FMT), incorporating the Harvie-Møller-Weare database; additions to the thermodynamic database, including the Pitzer parameters and standard chemical potentials, were derived from solubility and speciation work carried out primarily at the Pacific Northwest National Laboratory, Forschungszentrum Karlsruhe, and Florida State University. Calculations used to support the Compliance Recertification Application incorporated the solubility limiting solid phases of: Am(OH)3 , ThO2, and KNpO2CO3 for An(III, IV, V). Am(III) was used as the model for all the trivalent actinides, Th(IV) was used for the tetravalent actinides, and Np(V) was the model for the pentavalent actinides. Solubility and speciation calculations for Pu considered only the trivalent and tetravalent oxidation states, with a probability of 0.5 assigned to the occurrence of each oxidation state. Calculations for Np considered the tetravalent and pentavalent oxidation states, whereas the tetravalent and hexavalent oxidation states were used in the U model.