Humic acids obtained from sewage sludge both before and after composting were characterized, and the influence of composting on the formation of more stable and polycondensed structures was studied. These humic acids showed an aliphatic character and although no great differences between humic acids from uncomposted and composted sewage sludge were observed, data from elemental analysis, E₄/E ₅ ratio, filtration through Sephadex G-100 gel and infra-red spectra seemed to demonstrate that more polycondensed structures and hence a more stable organic matter, was obtained with the composting process.