A carbon dioxide (CO 2 ) injection pilot project is underway in Section 205 of the McElroy field, West Texas. High-resolution crosswell seismic imaging surveys were conducted before and after CO 2 flooding to monitor the CO 2 flood process and map the flooded zones. The velocity changes observed by these time-lapse surveys are typically on the order of -6%, with maximum values on the order of -10% in the vicinity of the injection well. These values generally agree with laboratory measurements if the effects of changing pore pressure are included. The observed dramatic compressional (V p ) and shear (V s ) velocity changes are considerably greater than we had initially predicted using the Gassmann (1951) fluid substitution analysis (Nolen-Hoeksema et al., 1995) because we had assumed reservoir pressure would not change from survey to survey. However, the post-CO 2 reservoir pore fluid pressure was substantially higher than the original pore pressure. In addition, our original petrophysical data for dry and brine-saturated reservoir rocks did not cover the range of pressure actually seen in the field. Therefore, we undertook a rock physics study of CO 2 flooding in the laboratory, under the simulated McElroy pressures and temperature. Our results show that the combined effects of pore pressure buildup and fluid substitution caused by CO 2 flooding make it petrophysically feasible to monitor the CO 2 flood process and to map the flooded zones seismically. The measured data show that V p decreases from a minimum 3.0% to as high as 10.9%, while V s decreases from 3.3% to 9.5% as the reservoir rocks are flooded with CO 2 under in-situ conditions. Such V p and V s decreases, even if averaged over all the samples measured, are probably detectable by either crosswell or high-resolution surface seismic imaging technologies. Our results show V p is sensitive to both the CO 2 saturation and the pore pressure increase, but V s is particularly sensitive to the pore pressure increase. As a result, the combined V p and V s changes caused by the CO 2 injection may be used, at least semiquantitatively, to separate CO 2 -flooded zones with pore pressure buildup from those regions without pore pressure buildup or to separate CO 2 zones from pressured-up, non-CO 2 zones. Our laboratory results show that the largest V p and V s changes caused by CO 2 injection are associated with high-porosity, high-permeability rocks. In other words, CO 2 flooding and pore pressure buildup decrease V p and V s more in high-porosity, high-permeability samples. Therefore, it may be possible to delineate such high-porosity, high-permeability streaks seismically in situ. If the streaks are thick enough compared to seismic resolution, they can be identified by the larger V p or V s changes.