In order to keep the anaerobic process stably and uniformly producing biogas it needs to be supplied with either an even amount of substrate of stable quality or varying amounts according to variations in quality. Feeding amounts are usually adjusted manually as a reaction to changing rates of biogas production. Continuous information about the actual substrate quality is not available and feedstuff analyses are costly. Aim of this study was to assess the feasibility of near infrared spectroscopic (NIRS) online monitoring of substrate quality in order to find ways towards more exact control of biogas plant feeding. A NIRS sensor system was designed, constructed and calibrated for continuous monitoring of (RMSECV in brackets) dry matter (DM) (0.75 %fresh matter (FM)), volatile solids (0.74 %FM), crude fat (0.09 %FM), crude protein (0.22 %FM), crude fiber (1.50 %DM) and nitrogen-free extracts (0.93 %FM) of maize silage. (C) 2011 Elsevier Ltd. All rights reserved.