Carbon Capture and Storage (CCS) has recently been gaining more and more attention as a climate change mitigation option. However, as CO2 may re-enter the atmosphere after injection into geological reservoirs, the question of long-term liability has to be considered if an environmentally sound policy is desired. Apart from this aspect, additional complexities arise from the fact that CO2 capture and storage can be carried out in two different countries. A classification of CCS cross-border activities shows that not all cases with non-Annex I participation fall under the Clean Development Mechanism. This classification is based on the assumption that according to Art. 1.8 of the Framework Convention on Climate Change, CCS would be considered an emission reduction at the source. Furthermore, we elaborate on the problem that seepage of CO2 from reservoirs located in non-Annex I countries – under current rules – would not be subtracted from the emission budget of any country. We discuss options for creating liability in these cases.