The effects of fresh and composted urban wastes on the organic matter of an arid soil were evaluated. The quality of the arid soil was improved with organic amendment. This improvement was more evident in soils amended with fresh residues (municipal solid waste and sewage sludge) than in those with compost. The latter, though having a less spectacular action, showed higher amounts of humic substances and fulvic and humic acids at the end of the incubation. The different carbon fractions of the soil amended with fresh residues showed significantly higher mineralized rates than with compost.