Alkali lignin as an example of waste plant biomass was subjected in a batch reactor to decomposition in sub- and supercritical water. Hydrothermolysis of alkali lignin with relatively highmolecular-weight led to the production of phenolic compounds such as guaiacol and catechol, as well as phenol, m-cresol, p-cresols and o-cresol. Both an increase growth in conversion time and an increase in reaction temperature promoted secondary reactions: degradation of alkali lignin decomposition products, leading to, their repolymerization and carbonization. The components of the liquid fractions of products were examined by GC-MS and HPLC, whereas the solid charred post-reaction residues were analyzed by elemental analysis and FTIR. (C) 2012 Elsevier B.V. All rights reserved.