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Factors that determine the segregation of soils belonging to the Mollisol or Aridisol orders have been evaluated in the semiarid-arid transition region of Argentina. The influence of edaphic parameters and precipitation on the soil organic carbon content (SOC) and color of the surficial horizons were studied on a transect of 300 kin length. Results indicate that the mean annual precipitation (MAP), the contents of clay (C), silt (Si), and their summation (C+Si), as well as the color attributes value (V), chroma (Ch) and their product (VCh) showed significant linear correlations with SOC (r=0.63, 0.55, 0.61, 0.65, -0.71, -0.45 and -0.66 respectively, p<0.01). These correlations, however, were of low predictive value (r(2) 0.20 to 0.50). A multiple regression model including the variables MAP and C + Si increased the explanation of the SOC variability up to 67% (p<0.001). The interaction between the climatic factor and the granulometric composition of the surficial horizon determine the content and transformation of SOC, allowing the segregation of mollic or ochric epipedons defining the taxon to which each soil belongs. With the MAP >400 turn, epipedons are mainly mollic, between 400-300 mm, the mollic or ochric epipedons are closely related to its texture, and in areas with MAP <300 mm, epipedons are predominantly ochric, due to its low SOC contents. Color attributes were the reason to exclude about 30% of surficial horizons from the Mollisol order, despite having SOC levels higher than that established by the definition of mollic epipedon. In the studied region, SOC and color attributes used as diagnostic criteria to assign mollic epipedons are not always congruent, and soil color becomes an essential variable to differentiate Mollisols from other taxa. When we grouped soil samples by granulometric similarity, C + Si explains up to 85% of the SOC variability. At the same time, if we limit the influence of different land uses, SOC storage capacity varies between 17 to 35 Mg ha(-1), increasing with higher precipitation and finer textures. (C) 2006 Elsevier B.V. All rights reserved.