The subcritical and supercritical water gasification of residue of some agricultural biomass samples (cotton and tobacco stalks) was investigated in a batch reactor at temperature range of 300-600 degrees C and experiments were performed by addition of 10 wt.% of different natural mineral catalysts (Trona. Dolomite and Borax). The yield and composition of gaseous, aqueous products and residue were identified by different analytical techniques. Gaseous products consisted mainly of permanent gases such as hydrogen, carbon dioxide, methane, carbon monoxide and C-2-C-4 compounds. Aqueous products consisted mainly of carboxylic acids (hydroxyacetic acid, formic acid and acetic acid), furfurals, phenols (phenol and cresols), aldehydes (formaldehyde and acetaldehyde). The effectiveness of three different natural mineral catalysts in SCWG can be classified as being Trona [Na-3(CO3)(HCO3)center dot 2H(2)O] > Borax [Na2B4O7 center dot 10H(2)O] > Dolomite [CaMg(CO3)(2)]. The results showed that the addition of Trona as catalysts at reaction temperature of 600 degrees C, hydrogen yields of tobacco and cotton stalks were increased by 42.9% and 39.9%, respectively. Crown Copyright (C) 2012 Published by Elsevier BM. All rights reserved.