The impact on soil properties of recent and ancient reforestations with Eucalyptus globulus Labill. has been studied in Cambisols and Leptosols under temperate semi-oceanic climate in Asturias (Northern Spain). Eucalypt forests showed a high potential of C accumulation in soil, mainly the ancient forest, where the organic matter concentration in the uppermost horizon (up to 415 C g x kg(-1)) was greater than in all neighbouring soil formations (climacic oak forests, up to 84 C g x kg(-1); Podsol under reforested pine forests, 287 C g x kg(-1); and Ulex shrub, 70 C g x kg(-1)). Nevertheless, there was a low concentration of available bases and a very low transformation degree of the organic matter in this eucalypt forest, only comparable to that of the Podsol. In order to analyse the major features of the biogeochemical behaviour of the sites under study, the humus fractions were isolated and the humic acid fraction was studied by derivative visible and infrared spectroscopies. Incubation experiments on whole soil samples were carried out to monitor stability against biodegradation of the organic matter under comparable conditions. In general, soils under eucalypt had a CO(2) production similar to or lower than the control sites, but the soil under the ancient eucalypt forest had a high potential for CO(2) release, similar to that of the neighbouring Podsol, what might be explained by the presence in these soils of high amounts of C accumulated in readily biodegradable forms. However, the mineralisation coefficients (CO(2) released per unit of soil C) indicated that the biodegradability of the soil humus has decreased significantly in the eucalypt-reforested sites, as it could correspond to an effective control of the biogeochemical processes caused by the antimicrobial products from the eucalypt litter. Compared to the original deciduous forests, the lower values of the E(4)/E(6) ratio in the humic acids from eucalypt forests and the lignin signature in the infrared spectra point to humification mechanisms based on the selective preservation of complex macromolecular substances derived from vascular plants. The second derivative of the visible spectra of the humic acid samples under study showed conspicuous valleys, which are ascribed to fungal-derived 4,9-dihydroxyperylene-3,10-quinone pigments. These valleys were in most eucalypt forests significantly shallower than in the original deciduous forests and may also represent an indicator of the impact of introduced vegetation on the structure of the soil microbial system and the mechanisms responsible for the formation of humic acids.