In order to investigate the effect of the antenna gain on land surface parameter retrievals with SSM/I, a Gauss-function is fitted to the 19 GHz antenna gain functions. Soil moisture and surface temperature distributions for the Red-Arkansas River basins are calculated from a soil-vegetation-atmosphere model. A passive microwave emission model is applied to derive polarized 19 GHz brightness temperatures. Those parameters are aggregated up to 19 GHz footprint size taking into account different spatial weighting according to the Gauss-function. These reference averages represent the full radiometer resolution, which is approximately three times the -3dB resolution. The comparisons between the reference values and simple linear averages for several spatial resolutions lead to differences varying from 2% to 17% volumetric soil moisture. ¿ 1999 American Geophysical Union