In the transportation of CO2 via steel pipelines in the CCS (Carbon Capture and Storage) process, a range of impurities may exist in the CO2 stream. Of the most important to the durability of pipelines with respect to corrosion damage, is the presence of an aqueous (water) phase. The reason for this is two-fold. Firstly such an aqueous phase permits corrosion and is also acidic owing to speciation of carbonic acid, while secondly, any other trace impurities which segregate to the aqueous phase may also enhance corrosion. Calculations within this paper indicate that the conditions likely to pertain in transport of CO2 will likely have an aqueous phase which exhibits stratified wavy flow predominantly along the bottom of the pipe. This flow pattern will be rapidly established and will not be destabilized by geometric features in the pipe. Contaminants such as HCl, HNO3 and SO3 will have a dramatic effect on the pH of the aqueous phase even in small concentrations while the impact of SO2 will be more moderate.