A new fractionation procedure using membrane ultrafiltration (UF), followed by chemical characterisation measurement of total organic carbon (TOC), chemical oxygen demand (COD) and organic nitrogen and spectroscopic study was applied to aqueous extracts of composting green wastes. Three membranes of molecular weight (MW) cut-offs of 1, 10 and 100 kDa were used. The study demonstrated the first step of the transfer of organic matter from the solids to the aqueous biofilm surrounding the solids. The microbiological consumption of the dissolved organic matter mainly used molecules smaller than I kDa, while the aromatisation of the organic matter, observed after 100 days composting, involved molecules larger than 10 kDa.