Humic acid (HA) and fulvic acid (FA) extracted from landfills at different landfill ages were characterized by elemental composition, (13)C CP/MAS NMR, and TMAH-Py-GC/MS. The elemental composition analysis revealed high O/C and low H/C ratios in the FA, indicating a high proportion of O-alkyl and carboxylic acids in the FA. The analytical results of (13)C CP/MAS NMR suggested that there were more oxygenated aliphatic carbons and fewer aromatic carbons in FA than in HA. The Py-GC/MS products showed that the HA and FA extracted from the refuse in the landfill were mainly composed of various lignin-derived compounds. Oxidized aromatic acid derivatives originated from the oxidation of side-chains of lignin-like compounds, and this process played a significant role in the process of HA and FA formation in the landfill. All of the results demonstrated that the degree of humification increased with landfill age.