Saline-sodic soils, which are both rich in salts and sodium, are degraded soils common in many parts of the world with subhumid to arid climates. This study was carried out to identify the main factors and processes related with the genesis under natural conditions of a saline-sodic soil (Sodic Haplusterts, fine clayey, smectitic, isohyperthermic) located in Tucupido (Guárico State, Venezuela), with a semiarid climate. There were evaluated morphological, physical, chemical and mineralogical characteristics and properties of the soil. The values of pH, exchangeable sodium and salinity (electric conductivity of the saturation extract) were 7.55-8.4; 2-32 % and 1.0-8.7 dS/m respectively. NaCl was the dominant salt (> 66 % below 0.1-m depth). The bulk density, the macroporosity and the saturated hydraulic conductivity were respectively 1.8-2.2 Mg/m³, 4-5 % and < 3x10^-7 m/sec. Smectites were the dominating clays, but chlorites and hydrated micas were also present. High quantities of Ca and Mg carbonates were also found precipitated in the soil (18 %). The conclusion is that those soils were probably formed by alteration of tertiary sedimentary rocks (calcareous shales), in contact with saline waters rich in sodium in the depressions of the Venezuelan Central Plains during the arid phase at the end of the Pleistocene period. The very poor permeability of these soils limited salts washing during the wetter Holocene period. The high electric conductivities and the high content of precipitated Ca carbonates probably prevented the increase of the exchangeable Na % in the soil to values higher than 32 %, despite the high content of NaCl. Collapse of interlayer space of 2:1 phyllosilicates (smectites) to prevent high exchangeable Na % was probably due to a high quantity of neutral salts (NaCl; incapable of undergoing alkaline hydrolysis) and to the production of Ca-Al silicate compounds which would be acting as cementing agents in the interlayer space of smectites.