A carbon based nickel (Ni) catalyst was prepared by ion exchange method in which a large amount of Ni is dispersed as almost uniform nano particles. The method ion exchanges the ion exchangeable sites in an ion exchange resin with Ni followed by carbonization at 500 degrees C. Two different catalysts were prepared by ion exchanging Ni from an aqueous solution containing same amount of Ni ion concentration but at different pH of the solution. The amount of Ni ion exchanged in both cases was about 15%. The metal load after carbonization was about 47% and 46% in the two catalysts, respectively. The XRD pattern and TEM images of the catalysts showed that Ni particles in NiWB500 (pH = 8.8) were bigger in size in comparison to the Ni particles in NiLG500 (pH = 9.4). The BET surface area was 178 and 183 m(2)/g, respectively. The catalytic hydrothermal gasification (CHTG) experiments at 350 degrees C, 20 MPa, and 50 h(-1) LHSV for 50 h with organic water containing 0.2% and 2% TOC concentration showed that conversion was almost 100% with NiLG500 (pH = 9.4) catalyst and 100% and 96% with NiWB500 (pH = 8.8) catalyst, respectively. The XRD and TEM patterns of the two catalysts after 50 h gasification run showed higher sintering in NiLG500 (pH = 9.4) in which Ni particles were smaller in size. (c) 2006 Elsevier Ltd. All rights reserved.