Improving the knowledge of anaerobic digestion process parameters can lead to a reduction of anaerobic digestion units' start-up phase with subsequent increase of their competitiveness. For this reason the start up phase of a pilot scale biogas plant fed with cattle manure underwent a six weeks fully monitoring during which we recorded daily and specific biogas and methane productions as well as process parameters like temperature of the biomass inside the fermenter, feeding amount, digestate pH and FOS/TAC value (the ratio between organic acids content, expressed as g m(-3) of equivalent acetic acid, and total inorganic carbon content of the fermenting biomass, expressed as g m(-3) of CaCO3). All data underwent multivariate statistical data processing based on principal component analysis (PCA) and linear discriminant analysis (LDA). At varying of the time required by the settling of biogas production (six weeks) different moments were differentiated and classified: according to multivariate analysis, among all the monitored process parameters, the FOS/TAC value was the one which better gave account of anaerobic digestion establishment and stabilization. (C) 2011 Elsevier Ltd. All rights reserved.