Passive microwave radiation measurements over sparsely vegetated surfaces during the Southern Great Plain Experiment are used to determine spatial up-scaling effects. For various distributions of geophysical parameters, the differences between mean soil moisture derived from high resolution (800 by 800m2) 1.4 GHz brightness temperatures and soil moisture calculated from mean brightness temperatures at larger scales are compared. For spatial resolutions of 31.42km2, the effects of these non-linearities in radiative transfer at 1.4 GHz do not yield differences in estimated volumetric surface soil moisture (0--5 cm) exceeding 0.02 [cm3cm-3>. ¿ 1999 American Geophysical Union