Four methods of monitoring the anaerobic digestion process were studied at pilot scale. The methods employed were Micro Gas Chromatography (mu-GC) and Membrane Inlet Mass Spectrometry (MIMS) for measurements in the gas phase, Near Infrared Spectroscopy (NIBS) and pH in the liquid phase. Micro Gas Chromatography accurately measured H(2), CH(4), H(2)S, N(2) and O(2) in the headspace whereas the MIMS accurately measured CH(4), CO(2), H(2)S, reduced organic sulfur compounds and p-cresol, also in the headspace. In the liquid phase, NIRS was found to be suitable for estimating the concentrations of acetate, propionate and total volatile fatty acids (VFA) but the error of prediction was too large for accurate quantification. Both the mu-GC and NIRS were low maintenance methods whereas the MIMS required frequent cleaning and background measurements. (C) 2010 Elsevier Ltd. All rights reserved.