A deterministic approach for downscaling 40km resolution Soil Moisture and Ocean Salinity (SMOS) observation is developed from 1km resolution MODerate resolution Imaging Spectroradiometer (MODIS) data. To account for the lower soil moisture sensitivity of MODIS surface temperature compared to that of L-band brightness temperature, the disaggregation scale is xed to 10 times the spatial resolution of MODIS thermal data (10km). Four dierent analytic downscaling relationships are derived from MODIS derived and physically-based model predictions of soil evaporative eciency (). The four downscaling algorithms dier with regards to i) the assumed relationship (linear or nonlinear) between and near-surface soil moisture, and ii) the scale at which soil parameters are available (40km or 10km). The 1km resolution airborne L-band brightness temperature from the National Airborne Field Experiment 2006 (NAFE'06) are used to generate a time series of eleven clear sky 40km by 60km near-surface soil moisture observations to represent SMOS pixels across the three-week experiment. The overall root mean square dierence between downscaled and observed soil moisture varies between 1.4% v/v and 1.8% v/v depending on the downscaling algorithm with soil moisture values ranging from 0 to 15% v/v. The accuracy and robustness of the downscaling algorithms are discussed in terms of their assumptions and applicability to SMOS.