The formation of lignin-based polyurethanes was monitored by Fourier transform infrared spectroscopy working in attenuated total reflectance mode (FTIR-ATR) and kinetics was analyzed according to a global second order model. The used chemical system consisted of 4,4'-methylene-diphenylene diisocyanate (MDI), polycaprolactone (PCL) of three different average-molecular weights (400, 750 and 1000) and two lignin (L) samples (Alcell (A) and Indulin AT (AT)), incorporated at different weight contents (10%, 15%, 20% and 25%) in the polyol mixture. The polymerization reaction was carried in bulk and without the presence of any catalyst. Results showed that isocyanate conversion decreases with the increase of both lignin content and PCL molecular weight. Moreover, the Indulin AT series leaded to lower isocyanate conversions compared to Alcell counterparts. The global second order treatment showed to be dependent of lignin type and content as well as PCL molecular weight. In fact, for the studied system this model was found to be more adequate for low lignin contents. After the region of linearity, a negative deviation was always observed and found to be more pronounced with increasing the lignin content. (C) 2011 Elsevier Ltd. All rights reserved.