The kinetics of soil organic matter (SOM) in cultivated tropical soils are characterized by rapid mineralization. The SOM level achieved in traditional cropping systems without long fallow periods is below the threshold generally agreed to be compatible with maintaining at acceptable level the soil productive functions. However, the SOM is made up of heterogeneous elements in terms of size, origin and chemical composition, and those different constituents can make very varying contributions to the properties of a given soil. So, knowing the kinetics of the SOM taken as a single unit is not enough to predict changes in soil properties linked to the SOM. Fractionating the soil according to particle size without destroying the SOM is one way of identifying the specific contribution of the SOM fractions to soil properties. This technique was applied to soils with and without manure or compost applications, from trials in Chad and the Ivory Coast. Five organo-mineral fractions were isolated: coarse sand, fine sand, coarse silt, fine silt and clay, comprising elements measuring between 2000 and 200, 200 and 50, 50 and 20, 20 and 2, and 2 and 0 µm respectively. Fractionation was carried out by mechanical and chemical dispersion of the soil, wet sieving of the fractions larger than 20 µm and sedimentation of the clay and fine silt fractions. Half the SOM in these tropical soils was of clay size. Compared to soils without organic applications, amendments proved to increase all the SOM fractions, although the coarse fractions were the most affected. When applied to the different soil size fractions from the Ivorian trial, the Hénin-Dupuis model revealed different kinetics for the SOM granulometric fractions, e.g. a half-life of two, eight and nineteen years for the SOM fractions of the size of sand, silt and clay respectively. Although its level was more stable, the clay-sized SOM fraction decreased also substantially in time without organic amendments. The CEC of the fractio