The thickness of soil layer, through which moisture can be directly estimated by means of a microwave radiometer, has been investigated experimentally on a test area in Central Italy by means of airborne sensors. Aircraft remote sensing data, collected on agricultural bare and vegetated fields during the growth stage of vegetation (May-July 1988), have shown that L band microwave emission is correlated to the average moisture of the first 20 cm of soil under the surface. However, correlation between moisture at difference depths makes the identification of the actual sampling depth difficult.