A technique to retrieve surface soil moisture was assessed at the global scale using a synthetic data set of L-band (1.4 GHz) brightness temperatures T-B for 2 years, 1987 and 1988. The global T-B database consists of half-degree continental pixels and accounts for within-pixel heterogeneity, on the basis of 1 km resolution land cover maps. The retrievals were performed using a three-parameter inversion method applied to the L-band Microwave Emission of Biosphere model ( L-MEB). Three land surface variables were retrieved simultaneously from the multiangular and dual-polarization T-B data: surface soil moisture wg, vegetation optical depth tau, and surface temperature T-S. The retrievals were obtained in two T-S configurations: T-S was either unknown or known with an uncertainty of 2 K. Applying these two assumptions, global maps of the estimated accuracy of the wg retrievals were produced, and the capability of the T-B to monitor wg was evaluated. A sensitivity study was carried out in order to analyze the effect of the main parameters that may affect the retrieval accuracy: the fraction cover of open water and forests, frozen soil conditions, and the radiometric noise on T-B. These results contribute to the better definition of the potential of the observations from future spaceborne missions such as the Soil Moisture and Ocean Salinity ( SMOS) project.