Airborne microwave measurements with a nadir-viewing X band radiometer operating at a wavelength of 2.65 cm are described. The measurements over adjacent bare and vegetated surfaces are compared with ground truth samples of soil moisture content (SMC). For the bare surface the emissivity is highly correlated (r=-0.97) with the SMC of the top 0.5 cm, with an antenna temperature dependence of -2.1¿K/% SMC. In contrast, the correlation over vegetated surfaces is very poor (-0.25). Thus it cannot be expected that the technique at this wavelength would be generally useful as a measure of SMC, and this is borne out by comparison with the Nimbus 5 electrically scanned microwave radiometer data. If aircraft or satellite radiometers are to measure SMC under vegetated conditions, it will be necessary to increase their wavelengths well beyond the X band.