A landfill bioreactor with leachate circulation in situ and two-phase landfill bioreactor were introduced to investigate the effect of acidification percentage and volatile organic acids on the decomposition of municipal solid waste. The concentration of soluble chemical oxygen demand (SCOD), volatile organic acids (VOAs), acetic acid, propionic acid and butyric acid in leachate were regularly monitored, and the acidification percentages were also calculated. The changes of SCOD indicated that the anaerobic landfill bioreactor with leachate circulation in situ occurred two clear tag-phases, the first one about 60 days and the second from the 80th day, while the two-phase one had the significantly short first lagphase, only about 7 days. It was also found that the concentration of VOAs and propionic acid could be used to indicate the first lag-phase and the second one, respectively. However, the percentage of acidification, which were similar for both lag-phase during the first 70 days, could only indicate the first lag-phase without showing the second lag-phase since the 80th day. (c) 2006 Elsevier Ltd. All rights reserved.