Reliable and cost effective monitoring techniques are required to ensure safe and effective geological sequestration of carbon dioxide (CO2), one of the promising strategies for mitigating CO2 emissions. This study proposes and examines a practical technique for monitoring the underground migration Of CO2 using Earth tides. The gravitational attraction of the bodies in the solar system causes tidal deformation of the Earth, and the pore pressure of the geological reservoirs changes in response to such tidal phenomena. With the pressure analysis algorithm adopted in this study, pressure fluctuations can be retrieved from the continuous pressure data obtained at the monitoring well. The diurnal and semi-diurnal features of the pressure fluctuations can be explained by the Earth tides, and it is possible to estimate the poroelastic parameter X, a function of the CO2 saturation in the pore space. By analyzing the X profile, CO2 migration can be monitored with a reasonable degree of accuracy. (c) 2005 Elsevier Ltd. All rights reserved.