Abstract Geological storage of CO2 is receiving growing attention by industry as an option to reduce CO2 emissions. For this, it is indispensable to develop geochemical models for the interaction of CO2 and incidental components like SO2 with formation rocks and brines. To establish such whole-rock models it is essential to have a good understanding of the chemical reactions, thermodynamic properties, and kinetic data of mono-mineral systems. In the German project “CO2 Purity for Capture and Storage (COORAL)” several institutes are working on the requirements for the purity of the CO2 stream during capture, transport, injection, and storage. Geochemical modeling and laboratory experiments on water-rock-gas interactions are carried out to improve understanding of mineral alteration processes. In whole- rock models of a generic Buntsandstein (sandstone), short-term precipitation of anhydrite occurred when SO2 was present. Furthermore, increasing feldspar dissolution and related illitisation reactions for the system CO2-SO2 in comparison to pure CO2 are of importance.