TX 75083-3836, U.S.A., fax 01-972-952-9435.Abstract CO 2 storage in saline aquifers may be used to reduce greenhouse gas emissions. After injection the CO 2 -plume migrates along the top of the aquifer for hundreds to thousands of years. An estimate of the area invaded by the CO 2 -plume is of interest for site selection. During plume migration, CO 2 is trapped as residual gas in the wake of the plume. Reduction of the mobile CO 2 volume by residual trapping would limit potential leakage.Plume evolution can be divided into an early stage, where the CO 2 has invaded the full thickness of the aquifer, and a late stage where the plume has spread enough to occupy only the top part of the aquifer. In the early regime we choose local variables for the dimensional analysis, to obtain a scaling law for the tip position x tip ∝ t 1/2 . At late time we choose global variables, obtaining a new scaling relation, x tip ∝ t 1/3 . Numerical simulations confirm the change in scaling for the tip position.We also discuss the effect of residual trapping on plume migration. We present semi-analytic results for a plume spreading in a horizontal aquifer and numerical results for a plume in a sloping aquifer. In the horizontal aquifer plume volume decays as a power law, so that the effi-ciency of residual trapping decreases with time. In the sloping case initial power law decay is followed by ever faster decrease in volume.The change in scaling for the CO 2 front has not been discussed before. This change from t 1/2 to t 1/3 scaling shows that the plume will spread significantly slower at late times, suggesting that the plume will stay more confined than early measurements would indicate. Residual trapping will be very effective in sloping aquifers, and may be the dominant trapping mechanism.