This work reports the development of a near-infrared (NIR) spectroscopy online calibration for monitoring volatile solids (VS) and total volatile fatty acids (VFA) process parameters during anaerobic digestion (AD). The objective was to investigate its potential to estimate the dynamics of AD process parameters after feeding events. A recirculation loop was employed to record online measurements during an eight-month experiment using a 3.3 m(3) pilot plant fed with maize silage under mesophilic conditions. Sampling was performed to conduct calibrations and subsequent test-set validations, comparing the NIR spectroscopy estimates to the reference values. The calibrated accuracy in terms of mean prediction errors (RMSEP) was 3 g kg(-1) for VS and 0.9 g kg(-1) for VFA in the fresh matter. By applying the calibrations to time series spectra, the model accuracy provided adequate indications of the concentration changes, including highly sensitive monitoring of short-term VFA dynamics. (C) 2012 Elsevier Ltd. All rights reserved.