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 TB for 2 years, 1987 and 1988. The global TB 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 TB data: surface soil moisture wg, vegetation optical depth τ, and surface temperature TS. The retrievals were obtained in two TS configurations: TS 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 TB 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 TB. 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.