Short-chain fatty acids (SCFAs) are the intermediate products of anaerobic digestion. The alkaline fermentation of primary sludge (PS) for ambient accumulation of SCFAs and mechanism were investigated in the present study. The results showed that the maximum SCFAs yields from PS fermentation was 312.9 mg COD/g VSS at pH 10.0-11.0 after 5 days reaction time, which was 1.8 times of that at neutral and acid pHs. The composition and distribution of SCFAs generated from PS anaerobic fermentation were studied, the maximum ratio of acetic, propionic, iso-butyric, n-butytic, iso-valetic and n-valetic acids was determined as 49.4%, 34.4%, 14.6%, 12.2%, 17.9% and 6.3%, respectively. In addition, the accumulation of SCFAs related with soluble organic compounds fermentation and methane production during PS fermentation was also investigated. The concentration of SCFAs generated from PS fermentation was found to be positive with the concentration of soluble organic compounds but negative with the production of methane. The accumulation of SCFAs at alkaline pHs can be attributed to the enhanced hydrolysis of PS at alkaline pHs which lead to more soluble protein and carbohydrate generation. At the same time, the activity of methanogens could be inhibited at alkaline pHs which resulting in less SCFAs consumption. (C) 2010 Elsevier B.V. All rights reserved.