Soils developed on the Tajo floodplain and on the alluvial plain of one of its main tributaries (Arroyo Melgar) have been studied between Aranjuez and Toledo (Central Spain). Generally soils are Typic xerofluvents but in local positions associated to abandoned channels or meanders, where hydromorphy processes take place, soils are Aquic xerofluvents. However in some cases, and under the same morphological conditions, a marked salinity occurs, dominating then Aquollic salorthids. High susceptibility to salinity in these environments results from three causes: the particularity of the geomorphological framework, the semiarid climate and the oscillations of the water table close to the surface (maximum depth 150-190 cm). The evaporation higher than the water recharge produces an upwards capillary movement favored by the fine textures of these soils. The horizons where salts and gypsum accumulates are developed in the intermediate zone of upwards-downwards soil solution movement. The results obtained show the possibilities of the application of geomorphological cartography in the diagnosis of the salinity, because this is a direct consequence of the combination of morphology, deposits, and drainage. The origin of this salinity is the result of the combination of local positions of soils profiles in deficient drainage units, the saline and gypsiferous Miocene substratum and the semiarid climate.