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Knowledge of the spatial distribution of global soil moisture for a long period of time is crucial to the understanding of climate changes and land surface processes. The observations from spaceborne passive microwave sensors for more than 30 years are very valuable for generating historical soil moisture maps. Except those from the Soil Moisture and Ocean Salinity (SMOS) mission, which was launched in November 2009, most of the observations available are from sensors with frequencies much higher than L-band and are more sensitive to vegetation conditions. To rebuild long-term soil moisture data sets with an improved accuracy, a method to correct vegetation effects at X-band or higher frequencies using L-band soil moisture retrievals is explored in this study. Validations based on b-parameter values derived from both field-sampled soil moisture with noises added and the actual SMOS products indicate that the method can be applied to the observations from the Advanced Microwave Scanning Radiometer for the Earth Observing System or similar sensors and help to rebuild historical soil moisture data sets with a root-mean-square error less than 0.04 m3/m3.