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The aim of this study was to establish the effect of parent material, climate conditions and vegetation type on soil organic matter (SOM) quality in different Mediterranean environments as an essential step for assessing soil stability; ecosystems in the first barrier against desertification in the western Mediterranean region. Thirty-six samples were taken from the organic-mineral horizon of unreclaimed soils with calcareous, acid metamorphic and calcareous/volcanic substratums; and from beneath three vegetation types (natural forests, reforested forests and scrublands) in areas of significant ecological value. The humus fractions and soil respiratory activity of bulk soil samples were studied, and some structural features of the humic acids extracted were characterized by elemental analysis and visible and infrared spectroscopies. Results indicate that SOM in the surface horizons of the calcareous and calcareous/volcanic areas showed higher biogeochemical transformation, where microbial synthesis in humic acids formation, including condensation mechanisms, prevails. In contrast, the acid metamorphic area showed the greatest differences, and would be considered more fragile in terms of organic matter stability to changes in the biogeochemical system. The results also indicate some differences due to the type of vegetation on soil humus chemistry. Under reforested forests of Pinus in an edaphic acid environment, humic acids showed a marked aliphatic character, displayed higher 2920 cm(-1) IR band, well defined typical lignin patterns, and higher E-4/E-6 ratio, i.e., accumulation of inherited macromolecular substances. The SOM beneath scrubland and natural Quercus forests may be said to be more decomposed (active breakdown of biomacromolecules) than beneath Pinus, humic acids exhibited higher aromaticity and were associated with accumulation of newly formed perylenequinonic chromophors of fungal origin, i.e., was at more advanced and complex stages of humification.