Abstract: The source and distribution of organic acids in Shenzhen municipal sludge and their relative stabilities during anaerobic digestion were investigated. Organic acids of bioorigin from domestic wastewater discharge and from indigenous microbe is found to be the main source responsible for the abundant organic acids present in the original sludge. The stability order of organic aids with different structure during anaerobic digestion is found to be: hydroxy acid < unsaturated acid < saturated acid < aromatic acid < steroid acid, which is roughly parallel to their chemical stability order, indicating that the anaerobic digestion of organic acids is mainly a structure-controlled degradation. Organic acids having aromatic ring, conjugated C = C bond and fused ring within their structure are normally more stable. Short chain hydroxy acids are selectively preserved against their long chain counterparts during anaerobic digestion. Anaerobic digestion may, under some circumstances be influenced by microbial metabolism, particularly the preference of carbon source.