During the last century attention has been given to the organic matter in tropical regions, principally in pedology studies. The Brazilian System of Soil Classification (SiBCS) (Embrapa, 1999) incorporated some attributes from international soil classifications that use some quantitative (organic matter content) and qualitative variables (fibers and degree of humification), especially for Spodosols, Molisols and Histosols, as in the Soil Taxonomy (USA). This demonstrates the importance of the organic matter in soil genesis and diagnostic horizons differentiation. The general objective of this study was to quantify organic matter fractions in some diagnostics horizons of Brazilian soils. For this project, 51 soil profiles were collected in various regions of the country and described according to the Brazilian Society of Soil Science and Embrapa standards. Soils were classified as Oxisols, Mollisols, Alfisols, Spodosols, Entisols, and Histosols. The organic matter fractions were obtained by differential solubility technical and the organic carbon quantified by oxidation with K₂Cr₂O₇ and titration with Fe(NH₄)₂.(SO₄)₂(6H₂O). In the Oxisols, the C in the total humic fractions showed the highest values among the soils studied, an average of 95.4%. The C-HAF/C-FAF ratio varied from 0.08 to 2.94 and alkaline extract (AE)/C-Humin varied from 0.32 to 1.07. For the Entisols, the C in the total humic fractions represented 80.7% of total carbon, where the sum of C-HAF and C-Humin represented 69.9%, indicating influence of drainage in the humification process, due to anaerobic conditions that control the organic fractions accumulation. The C-HAF/C-FAF ratio varied from 0.71 to 3.39, with most ratio values superior to 1.0. Different behavior was observed for the AE/C-Humin ratio, whose values varied from 0.36 to 1.43, however the majority of ratio values were inferior to 1.0. For the Histosols, it was observed the smallest average value of C in the humic fraction, around 62.0%, and the sum of C-HAF and C-Humin an average of 55.6%, with predominance of C-HAF. These results indicate the influence of drainage in the humification process. The C-HAF/C-FAF ratio had a large variation, with values ranging from 1.91 to 16.94. The same was observed in the AE/C-Humin ratio, where values varied from 0.41 to 7.24, and the majority of values were lower than 2.0. In the spodic horizons, the C in the humic fractions represented 74.4% of total organic carbon. The C-FAF and C-HAF sum represented 60.0% of total organic carbon. As in the other soil orders the C-HAF/C-FAF ratio showed a wide variation, with values varying from 0.06 to 1.48, and AE/C-Humin ratio values ranged from 1.48 to 24.8. For the Molisols, C in the humic fraction represented 80.2%, where the Humin showed the highest values (64.7%), compared to the other fractions and among the soils studied. The C-HAF/C-FAF ratio varied from 0.21 to 3.50, and AE/C-Humin ratio showed the smallest variation, with values ranging from 0.17 to 0.35. The results indicate that the different pedoenvironments influence the distribution of organic matter fractions, which demonstrate the importance of this attribute in soil classification, especially for the soil orders studied. Thus, it is proposed that the fractioning of soil organic matter becomes criteria to soil classification, at the family or series level.