The non-isothermal pyrolysis kinetics of Acetocell (the organosolv) and Lignoboost (R) (kraft) lignins, in an inert atmosphere, have been studied by thermogravimetric analysis. Using isoconversional analysis, it was concluded that the apparent activation energy for all lignins strongly depends on conversion, showing that the pyrolysis of lignins is not a single chemical process. It was identified that the pyrolysis process of Acetocell and Lignoboost (R) lignin takes place over three reaction steps, which was confirmed by appearance of the corresponding isokinetic relationships (IKR). It was found that major pyrolysis stage of both lignins is characterized by stilbene pyrolysis reactions, which were subsequently followed by decomposition reactions of products derived from the stilbene pyrolytic process. It was concluded that non-isothermal pyrolysis of Acetocell and Lignoboost (R) lignins can be best described by n-th (n > 1) reaction order kinetics, using the Weibull mixture model (as distributed reactivity model) with alternating shape parameters. (C) 2011 Elsevier Ltd. All rights reserved.