Modifications of the lipidic and humic acid-like fractions from two organic wastes (city refuse and aerobic sewage sludge) before and after a composting process were studied. Although the qualitative composition of the lipidic fractions before and after composting was nearly identical, some quantitative differences were observed. Data from elemental and functional group analysis and FTIR spectra of the humic acid-like fractions showed a slight increase of the aromatic compounds during the composting process. GC-MS studies of the products of the persulfate and permanganate oxidation of the humic acid-like fractions have not been useful to show changes in them. I NTRODUCTI ON Applications of different organic amendments to crop land has been a common practice in agriculture for a long time. These organic amendments cannot be substituted by inorganic fertilizers and nowadays the important role of the organic matter in soil formation, soil fertility and crop production has been widely established (refs. l-2).The intensive cultivation practices facilitate rapid degradation of organic matter. Hence it is necessary to use increasing amounts of organic fertilizer to mantain or to improve soil fertility. Organic wastes such as sewage sludges or city refuse can be used for applications in agriculture or horticulture if they have been composted to avoid toxicity problems caused by not stabilized inorganic and organic constituents. These problems appear when raw wastes or compost with a low degree of maturity are used (refs. 3-4). A more complete knowledge sf the compost organic fractions is necessary for the understanding of some important agronomic effects of the refuse compost applications. Although the compost humic-like fraction is defined in operational terms in the same way as the soil humic and fulvic acids, taking 0048-9697/89/$03.50 © 1989 Elsevier Science Publishers B.V.