Increases in pore pressure and volume caused by CO2 injection will perturb the ambient stress field and possibly trigger brittle failure on small fractures or faults, resulting in microearthquakes. Monitoring this induced seismicity can help understand the migration of the CO2 front, and the stress and strain changes in the reservoir (Miyazawa et al., 2008). As part of research efforts of the Southwest Regional Partnership on Carbon Sequestration supported by the U.S. Department of Energy and managed by the National Energy Technology Laboratory, a large number of microseismic events have been recorded since early 2008 using a geophone string cemented into a well for monitoring CO2 injection at the Aneth oil field in Utah. We use double-difference tomography to obtain high-precision locations of microseismic events and improve the spatial resolution of the velocity structure simultaneously. The double-difference seismic tomography method was developed for improving earthquake source locations and tomography imaging (Waldhauser and Ellsworth, 2000; Zhang and Thurber, 2003; Okada et al., 2006). The double-difference method uses both the absolute and differential arrival times to invert for the velocity distribution and the source locations simultaneously. Because catalog and cross-correlation data are combined into one system of equations, interevent distances within multiplets are determined to the accuracy of the cross-correlation data, while the relative locations between multiplets and uncorrelated events are simultaneously determined to the accuracy of the absolute traveltime data. From the studies conducted by Waldhauser and Ellsworth, the uncertainties in double-difference locations are improved by more than an order of magnitude compared to catalog locations. Zhang and Thurber tested the double-difference tomography on a synthetic data set and found that it produced a more accurate velocity model and event locations than standard tomography.