This paper reports the results of experiments performed on the flash pyrolysis of lignin samples submitted to controlled heat flux densities (short flashes of a concentrated radiation). Two types of lignins are used: Kraft and Organocell lignins. Microscopic observations of the reacted samples reveal the formation of an intermediate liquid compound that precedes the further formation of char, vapours and gases. The rates of mass loss and the production rates of the products are determined for both lignins. The results are compared to each other and to those obtained in former similar studies made with cellulose. The analyses of the produced gases reveal high syngas and H-2 contents (respectively 87 and 50 mol%). This composition is compared to results obtained in other different thermal conditions with lignins and other types of biomasses. The possible mechanism of hydrogen formation is further discussed. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.