This report presents the results of Phase II of the NEA Sorption Project, which was initiated as a major international contribution towards demonstrating the consistency and applicability of different thermodynamic sorption models to support the selection of a sorption parameter, namely Kd values, for safety assessments. It was implemented in the form of a comparative modelling exercise based on selected datasets for radionuclide sorption by both simple and complex materials.  The project was conducted under the auspices of the Integration Group for the Safety Case (IGSC) of the OECD/NEA Radioactive Waste Management Committee (RWMC).