An understanding of the behavior of land surfaces at microwave frequencies is important for the purpose of rainfall estimation. This paper presents the results of the RISMOP experimental campaign (radiometric impact of surface moisture after precipitation). In an initial ground experiment soil and vegetation samples were observed from a height of 2 m at 23.8 and 50.1 GHz and at both horizontal and vertical polarizations. The observations were made under both dry and wet conditions. The emissivity of soil was observed to decrease by up to 0.35 between dry and saturated soil conditions. The Met Office research aircraft made passive microwave measurements over Central Spain at 23.8, 50.1, 89 and 157 GHz. The flights were coincident with ground observations of soil moisture and cover type. Three of the flights took place under dry conditions and the fourth occurred shortly after rainfall. The flight data indicates an increase in surface emissivity with vegetation cover of up to 0.1 under dry conditions and 0.2 under wet conditions. Significant decreases in surface emissivity were observed between dry and moist soil conditions.