Subsurface storage of carbon dioxide (CO2) is regarded as a short to medium term solution for reducing greenhouse gas emissions. However, there are concerns with respect to the integrity of seals in subsurface storage of CO2 and the risks associated with leakage to ocean and atmosphere. In this paper,wereport the results of experimental laboratory simulation of CO2 leakage from subsurface storage sites and the selfsealing mechanism of CO2 hydrates in subsea sediments, using an experimental setup specifically constructed for this work. The results demonstrate that the sequestrated CO2 migrated upward and formed hydrates with the pore water in the sediment whenthe pressure and temperature conditions in the sediments were inside the hydrate stability zone. The CO2 hydrate formation slowed down the CO2 diffusion rate by several times to 3 orders of magnitude. The upward migrating CO2 tended to form hydrate at the base of the hydrate stability zone. On the geological time scale the CO2 hydrate formation could create a low-permeability secondary cap layer which greatly restricts further upward CO2 flow,shouldaleakage occurs. This potential "self-sealing" and "self-healing" process could be an important criterion in the selection of suitable sites for geological storage of CO2. © 2010 American Chemical Society.