Completely mixed batch reactors were used to compare the methane production from maize at inoculum to substrate volatile solids ratios (r(l/s)) of 3, 2, 1.5 and 1. The tests were carried out at 35 degrees C and run against a control of inoculum without substrate. The stability and progress of the reaction from solid substrate to gaseous end product was monitored by measuring the pH, soluble chemical oxygen demand (CODs), total alkalinity (TA), volatile fatty acids (VFA) and gas composition on a daily basis. The results showed that the yield coefficient showed little variation over the range used, with an average value of 211 +/- 6 ml CH4 at standard temperature and pressure (STP) conditions g VSadded.-1 The methane added production curves were further analysed to give values for the maximum specific methane production rate, ranging from 10 ml CH4 a. VSS-1 day(-1) for a r(l/s) of 3-23 ml CH4 g VSS-1 day(-1) for a r(1/s) of 1. The initial buffering capacity of the system was enhanced by addition of NaHCO3 to 9 100 mg CaCO3 1(-1) which allowed digestion to proceed within the pH range 7.2-8.0 and at TVFA to TA ratio below the critical limit of 0.4. The largest proportion of the CODs was in the form of VFA and the reactor with the highest VS loading showed an accumulation of longer chain acids. (c) 2006 Elsevier Ltd. All rights reserved.