Geocenter variations are caused by mass redistribution within the Earth system, especially the atmosphere, oceans, and continental water. Using surface pressure fields, and soil moisture and snow depth fields of the NCEP-NCAR Climate Data Assimilation System I (CDAS-1), we estimate contributions from variations in atmospheric surface pressure and continental water storage to the Earth's geocenter (center of mass) variation. In addition, sea surface anomalies determined by the TOPEX/POSEIDON altimeter are used to investigate geocenter variations resulting from ocean mass redistribution. These sea surface height data were corrected using a simplified steric model. A comparison with observed geocenter variations derived from Lageos 1 and 2 satellite laser ranging data indicates that the atmosphere, oceans, and continental hydrological cycle all provide significant contributions at different frequencies. Geocenter variations estimated in this paper are in reasonably good agreement with results given by Dong et al. <1997> for atmospheric and ocean contributions, but not for the estimates of continental hydrological contributions. ¿ 1999 American Geophysical Union