【Introduction】 The most widely distributed soil in the Middle Amazon is the yellow latosol. However, a dark soil with a fertile surface known locally as Terra Preto do Indio or Amazonian Dark Earths (ADE) is found scattered in many places of the Amazon basin (Sombroek, 1966). Recently, it has been shown that the surface of the ADE is characterized by high phosphate content, exchangeable Ca, humic substances, and anthropogenic ceramics. Additionally, the distribution of ADE sites corresponds to prehistoric settlement areas (Lehmann, et al., 2003). These facts seem to indicate that soil amelioration was accomplished by prehistoric Amerindian population. To this date, however, a detailed pedogenesis process of ADE has not been carried out to clarify the connection. One of the major problems of sustainable agriculture in the humid tropics is the rapid decomposition of soil organic matter due to the high temperature and precipitation, and the absence of stabilizing minerals. A notable fact about the ADE is that, in contrast to surrounding soils, it is able to maintain a higher content of soil organic matter. In particular, the accumulation process of soil organic matter in ADE is one of the most important issues regarding its genetic process. The primary objective in this study was to understand the composition of humic substances in ADE by the NAGOYA method. 【Materials and Methods】 Soil samples were collected from three settlements, Site 1 (Vila Terra Preta, VTP), Site 2 (Cajual, CJ), and Site 3 (Bom Futuro do Maranhão, BFM), located along the black water Uaicurapá River, to the south of Parintins, Amazonas State, Brazil. The soil samples were analyzed by the NAGOYA method (Kumada, et al., 1967). The NAGOYA method determines the type of humic acid based on its amount and optical density. First, 30 ml of 0.1 mol/L NaOH were added to 2 g samples (containing less than 100 mg organic C), and heated in boiling water (100℃) for 30 min, shaking the flasks every 10 min. After the procedure, 1g of Na₂SO₄ was added to the flasks as a coagulating agent, and the samples were cooled in ice water bath, and centrifuged at 11,000 rpm for 15 min. We decanted the alkaline extract and washed the soil residue twice with a 20 ml extractant containing Na₂SO₄ through centrifugation as before. The combined extract and the washed residues were acidified with conc. H₂SO₄ (1 ml/100ml), and they were left to stand for 30 min. Soil residues were transferred back to the centrifuge tube with 30 ml of 0.1mol/L Na₄P₂O₇ and treated again with Na₄P₂O₇, instead of NaOH. Both acidified extracts, i.e., NaOH and Na₄P₂O₇, respectively, were filtered with filter paper (Toyo Roshi No. 6) into a volumetric flask, and washed to precipitate with H₂SO₄ (1:100) and made the volume of filtrate (fulvic acid fraction) to 100 ml with H₂SO₄ (1:100). The precipitates were dissolved (humic acid fraction) with 0.1 mol/L NaOH in 100 ml volumetric flasks (depending on HA content). Following that, the absorption spectrum of humic acid was measured in the range of 220-700 nm within 2 hr after dissolution. The amount of humic acid and fulvic acid fractions were determined by acid permanganate oxidation. Alternatively, the amount of Fe, Al, and Ca in each fraction (humic and fulvic acids) was determined by Inductively Coupled Plasma Spectrometry (ICP), after the organic C was degraded by hydrogen peroxide. 【Results and Discussion】 The amount of humic acid in ADE showed distinctly higher values in contrast to adjacent yellow soils. The optical properties of humic acid indicated that its RF value did not always show the highest value on the surface layer of the ADE. It was shown that the darkest black humic acids often exist in the AB or BA horizon. The humic acid extracted with NaOH showed the A type in seven horizons, which is a common trait in the ADE. The type of humic acid extracted by Na₄P₂O₇ showed the A type in all dark colored horizons. Moreover, the soil horizon with the humic acid extracted by NaOH depicting the A type, indicated high values of Fe and Al compared with other layers. It is now known that the black colored layer contains much soil organic matter, in relation to adjacent yellow latosol. Glaser, et al. (2001) demonstrated that the ADE contains large amounts of residues resulting from the incomplete combustion of organic material (black carbon, BC). We concluded that the organic matter of ADE, possibly originating from BC, form a mineral organic complex with Fe or Al, and generate the A type humic acid that take on the black tint and stability. As a result, ADE maintains the black humic substances highly stable.