Since October 1996, Statoil and its Sleipner partners have injected CO2 into a saline aquifer, the Utsira Sand, at a depth of approximately 1000 in. The aquifer has a thickness of more than 200 in near the injection site and is sealed by thick shales. A multi-institutional research project SACS (Saline Aquifer CO2 Storage) was formed to predict and monitor the migration of the injected CO2. To this end two time-lapse seismic surveys over the injection area have been acquired, one in October 1999, after 2.35 million tonnes Of CO2 had been injected, and the second in October 2001, after approximately 4.26 million tonnes of CO2 had been injected. Comparison with the baseline seismic survey of 1994 prior to injection provides insights into the migration of the CO2. In this paper the results of the seismic interpretation will be shown, supported by synthetic seismic modelling and reservoir flow simulation of the migrating CO2 at the two different time-steps. (C) 2004 Elsevier Ltd. All rights reserved.