In the context of carbon capture and storage (CCS), quantitative estimation of injected CO2 is of vital importance to verify if the process occurs without any leakage. From a geophysical perspective this is challenging as a CO2 plume has a severe imprint on seismic data. While this makes delineation of the plume rather straightforward, for quantitative interpretation a technique is required that takes complex wave propagation, including multiple scattering and mode conversions into account. In this abstract a wave-equation based AVO technique is discussed and successfully demonstrated on a seismic dataset from the Sleipner site. The technique solves the exact wave-equation which means that tuning effects are properly modelled. The scheme directly inverts for compressibility and shear compliance as these parameters are more closely related to saturation than conventional impedances. From this the total amount of injected CO2 is calculated and found to be in good agreement with the known value at the time when the data was acquired. © 2018 European Association of Geoscientists and Engineers, EAGE. All rights reserved.