Changes in amount and characteristics of humic acids (HA) were studied during the composting of a biowaste in an 80 1 composting device. HA was extracted by NaOH, purified with HCl-HF and dialysed. Freeze-dried HA was characterised by UV-285, elemental analysis, C-13 NMR and pyrolysis-GC/MS. Composting of biowaste resulted in the degradation of 65% df the organic matter. The amount of HA decreased in the initial stage of composting but started to increase again after 20 days. At the start, HA was mainly composed of aliphatic compounds which were replaced by aromatic compounds during composting. Characterisation of HA revealed that the contribution of the condensation route was significant for HA formation during composting. Maturity indexes based on NaOH-extracted HA were inappropriate and moreover, beneficial effects of compost amendments to soils cannot be studied by organic matter analysis of bulk compost, but the amount and characteristics of available HA should be determined.