The nickel-impregnated rice straw (RS) was gasified in steam/argon flow to examine the influence of difference in chemical compositions on the hydrogen formation. The weight ratio of cellulose:hemicellulose:lignin is approximately 3:3:2. For the RS without nickel, the hydrogen began to be evolved from around 700 degrees C and the hydrogen evolution reached a maximum rate at 900 degrees C. By loading nickel catalyst, a new peak appeared at 700 degrees C in addition to the original peak at 900 degrees C. The hydrogen yields increased slightly with the amount of nickel. In order to change the chemical composition of rice straw significantly, the RS was treated hydrothermally at 220 degrees C for 5 min, resulting in the weight ratio of cellulose:hemicellulose:lignin being approximately 4:2:1. Hydrogen evolution profile of the hydrothermal residue of rice straw (RSR) without nickel was similar to that for the RS without nickel. By loading nickel, however, the hydrogen yields increased considerably. The difference in the behavior of hydrogen evolution between RS and RSR with nickel was discussed on the basis of the difference in their chemical compositions. (C) 2011 Elsevier B.V. All rights reserved.