Soil Organic Matter (SOM) is an important factor in regional and global C budgets because it serves as a sink of large amounts of Organic C (OC) and nutrients enhance on it. The amount and quality of soil C provides a basis to evaluate its potential resilience. In the mountain range areas of the Sierra Sur de Oaxaca (SSO) changes in soil OC-storage may be the result of land use and degradation, due to environmental perturbations associated with steep slopes. This region was covered by sub deciduous medium forest, which is a natural reservoir of soil and forest resources; since S. XIX coffee plantations were introduced as rustican agroforestry structure, similar to the original forest conditions that represent the highest potential for sustainable soil conservation. With the aim of characterizing the SOM stability by the humic substances composition in several soils representative of the region, samples of topsoils previously classified as Humic Acrisols, Humic Umbrisols, Mollic Cambisols, Haplic Luvisols, Haplic, Skeletic and Epileptic Calcaric Phaeozems and Umbric Rhodic Ferralsol, were taken from Los Santos Reyes Nopala, Pluma Hidalgo and Santa Maria Huatulco localities in the Pacific steep-slope of Oaxaca State, between 1400 – 800 masl. Humus fractions were isolated by sequential fractionation, as Ponomareva and Plotnikova (1980), Duchaufour and Jacquin (1975), Monnier et al. (1962) and Dabin (1971) methods. The quantitative proportion (in C) of the humic substances fractions was determined by the Walkley & Black method. The UV-VIS spectra were obtained as Belxchikova and Kononova (1960) and absorbance relations E4:E6. FTIR spectra were obtained from 4000-400 cm-1 in a Bruker IFS28 spectrophotometer. Both spectra were enhaced as Rosenfield & Kak, 1982. Data were analyzed with Statistica V.6.0 and Statgraphics V.3.2. In Haplic Phaeozems, a high proportion of C was extracted by the alkaline solutions at pH 7, increasing again with 0.1M NaOH-Na4P2O7, mainly for the fulvic acid (FA) fraction, suggesting that there is a high proportion of complexes with metals and clays in these soils. This assumption is consistent also with the highest proportion of C obtained as humin (HUM). In Humic Acrisols HA:FA ratios 0.62 and 0.76, indicate a higher content of FA and Hum for the 0-20 cm and 20-40 cm samples respectively. HAs VIS-UV spectra and their E4:E6 ratios varying from 4.58 to 5.43, are not statistically different, and show a very low correlation with their steep-slope position (r-0.43, p<0.05). In general, there is a decrease of the VIS-UV ratios as an index of their higher aromaticity, after obtaining the absorption maxima at 615-620 nm more evident in the Mollic Cambisols, corresponding to the melanic substances of dihidroxyperilenquinone (DHPQ) type, produced by fungi during the HAs formation, that has been ascribed by Kumada and Hurst (1967) as the green fraction of HA Type P, also described by Watanabe et al. (2001) in Cambisols. The DHPQ is less evident in the Humic Acrisols. FTIR spectra of humic acids (HAs) shows signals at 2920 cm-1 that is a signal of the predominance of aliphatic structures and at 1510, 1460, 1420, 1380, 1270 and 1230 cm-1 corresponding to lignin patterns. The ratio 2920:1510 is >1, also indicate a high content of C aliphatic compared with the C aromatic structures. Humic Umbrisols have also a higher content of FA fraction with HA:FA ratios of 0.98 to 0.976 in the surface layer, and 0.598 to 0.692 in the subsurface samples, HUM percentage varies between 50.4 to 54.8 at 0-20 cm and from 49 to 69.2 in the subsurface layer. Mollic Cambisols have ratios of HA:FA 0.874 at 0-20 cm and 0.792 at 20-40 cm, humin fraction also has the higher OC content between 48.5 to 57.1, respectively. In Skeletic Phaeozem, light organic matter (LOM) dominates in the composition of humus, the main part of the extractable HS consisted of FA, with HA/FA ratios of 0.27 and 0.55, indicates that the bulk of OM is only weakly bound to the mineral surfaces, also humin fraction C content was lower than in the other soils, all these parameters are related to their fragility. In the Umbric Rhodic Ferralsol, LOM is very low, HUM constituted almost a half of SOM. The majority of extractable HS are represented by fulvic acids, with a HA/FA ratio of 0.28. That indicates a moderate degree of C stability. In Epileptic Calcaric Phaeozem, HA fraction is the main component of extractable HS, Humin constitutes more than a half of OC, related with the physical and chemical stabilization of the SOM. High proportion of the humin fraction may be due to the complex origin and diagenetic processes of the soil humus fraction, (Almendros et al, 1991). These differences in soil properties result in different ecological functions of the soils.