The hydrothermal reactions of vanillic acid in water near the critical point have been investigated in a continuous tubular reactor at temperatures between 280 and 500degreesC and pressures between 225 and 300 bar. Formation of 2-methoxy-phenol as primary product through decarboxylation of vanillic acid is the main decomposition reaction. Decarboxylation kinetics is first-order on vanillic acid concentration. The reaction rate constant is independent of pressure below the critical temperature and decreases with pressure at supercritical temperatures. For instance, at 350 degreesC it was 0.127 +/- 0.006 s(-1) at 250 bar and 0.126 +/- 0.014 s(-1) at 300 bar, while at 425 degreesC it was 0.694 +/- 0.042 and 0.505 +/- 0.024 s(-1) at the same pressures, respectively. The effect of pressure on reaction rates above the critical temperature is discussed on the basis of a reaction mechanism that involves an activated state with a positive volume of activation, which corresponds to an unimolecular dissociation. The secondary products formed by decomposition of 2-methoxy-phenol are also considered. (C) 2003 Elsevier B.V. All rights reserved.