We used the pyrolysis-gas chromatography technique (Py-GC) to characterize mature and immature soil organic matter (PN soil: C/N = 24.8; PS soil: C/N = 12.6). PN produced on pyrolysis an abundance of furfural and phenol, indicating the presence of partially decomposed litter. PS, on the contrary, produced a considerable amount of benzene, which was consistent with a more mature humus. The extracts were fractionated in HAs and FAs and by membrane ultrafiltration, in fractions A1(mol wt > 105), A2(>104), and R (<104). Extracts and relative fractions were characterized by Py-GC and isoelectric focusing (IEF). The elaboration of the relative abundances and several Py-GC ratios proved to be very appropriate in assessing differences or similarities among soils and their fractions. Independently of the maturity of soil humus, the A1s and HAs were carbohydrate-rich and composed of highly condensed humus; the Rs were polyphenol-rich, less condensed, and similar to FAs; the A2s showed intermediate properties. IEF has proved a reliable technique for characterizing structurally different organic fractions. The IEF patterns of the fractions were charge-size depenedent, and, on the basis of the charge and heterogeneity, they supported the Py-GC results. © 1986 The Williams & Wilkins Co.