It is widely recognized that carbon dioxide (CO2) is an important agent in global warming, and much recent activity has focused on finding ways to reduce, eliminate, or sequester CO2 in order to minimize its impact on the environment. In addition, miscible CO2 floods have become increasingly important as an EOR (enhanced oil recovery) method for recovering residual or bypassed oil, and as an effective mechanism for long-term storage of CO2. Clearly, an understanding of the effects of CO2 on rock-fluid systems, together with an ability to accurately map CO2 fronts during injection, are crucial for improving recovery rates, optimizing well patterns, locating bypassed oil and minimizing the cost of injected CO2. This paper describes a new method to predict and quantify the effects of CO2 injection in porous saline and oil/brine reservoirs using time-lapse seismic data. Unlike many inversion methods, this model-based approach is flexible enough to invert for multiple reservoir properties and their uncertainties using data that may include seismic tuning and other potentially complicating effects.