Organic matter from different soil size fractions (<2 μm; 2–20 μm; 20–50 μm; 50–150 μm, and > 150 μ) was obtained, using a combination of physical separation and light chemical treatment, in an Argentine Mollisol. Quantification of semiquinone free radicals was done using electron spin resonance spectroscopy, taking into account the organic carbon content of each sample. Values obtained for fractions less than 50 μm in size were significantly higher than values for fractions greater than 50μm. Comparison of this data with C/N ratios suggests that there is an association between fractions where more humified organic matter occurs and a higher level of semiquinone is detected. A 2 to 20-μm size fraction was identified as that with the highest humification degree and one which undergoes only minor changes as a result of agricultural practices. Alterations associated with soil compaction, principally in an A12 horizon in an area cultivated for 80 years, were accompanied by a reduction in the level of semiquinone free radicals compared with a native grassland area. Otherwise, in an area with only 10 years of soybean monoculture and where soil compaction effects are not yet extensive, a strong increase of free radicals was observed, principally in the Ap horizon. This increase of free radicals is associated with higher oxidative conditions of a relatively new cultivation area and also with probable rapid decomposition of soybean residues and incorporation as humified organic matter.