Every soil has a limited capacity to physical-chemically protect organic matter from biodegradation in its clay and silt particles. Thus, the rates at which carbon (C) and nitrogen (N) are mineralized in soil should be related to the degree of C saturation in those particles. In this study the authors investigated the hypothesis that the decline in the amount of C in clay and silt particles from coarse to fine textured soil is due to a reduction of the specific surface area of clay and silt fractions in clay soils. The relation between the mineralization rates of C and N to the degree of C saturation in the clay and silt particles was also investigated. Six cultivated soils (cereal) and three non-cultivated soils (Medicago sativa L.) were sampled at 0-20 cm depth. All soils were ultrasonically treated and gravimetrically separated into fractions > 50 µm (sand) and < 50 µm (clay and silt). All fractions and whole soils were analyzed for organic-C and the specific surface area with ethylene glycol monoethyl (EGME). Moreover, there was a close relation between the degree of C saturation and the mineralization rates. However, there also was a good relation between the total organic C of the soil and the mineralization rates. This coincided with the fact that a good relation between the soil organic carbon and the degree of carbon saturation was found. Our results partially explain the hypothesis that the mineralization rates are more related to the degree of C saturation than to the soil texture (% clay and silt) and soil organic carbon.