The lignin gasification in supercritical water with 20 wt.% Ni/MgO catalyst showed high catalytic performance, however, the deactivation of catalyst was observed when this catalyst was reused successively three times. The used catalyst was characterized by BET, XRD, TPR, CO chemical adsorption techniques, TPO and TEM observation, and the reasons of deactivation were discussed in detail. When the mixture of Ni/MgO catalyst and THF insolubles obtained in the previous run was used as a catalyst for the next run, the carbon yield of gas products considerably dropped due to the presence of char-like carbonaceous products and the formation of Mg(OH)(2) Phase. Moreover, the deactivation of catalyst was also observed even after used catalyst was regenerated by oxidation at 1023 K for 1 h and reduction at 1173 K for 1h. The decrease in Ni metal surface area of catalyst treated by oxidation and reduction probably caused catalyst deactivation. The deactivation of catalyst during reuse test with Ni/MgO catalyst occurred regardless of regeneration treatments. (c) 2007 Elsevier B.V. All rights reserved.