The effect mechanism of calcium oxide (CaO) addition on gasification of pyrolytic volatiles as a key sub-process in the absorption-enhanced steam gasification of biomass (AESGB) for H-2 production at different conditions was investigated using a two-stage fixed-bed pyrolysis-gasification system. The results indicate that CaO functions as a CO2 absorbent and a catalyst in the volatiles gasification process. CaO triggers the chemical equilibrium shift to produce more H-2 and accelerates volatile cracking and gasification reactions to obtain high volatile conversion rates. Increasing the gasification temperature could improve the reaction rate of cracking and gasification of volatiles as well as the catalytic effect of CaO, which continuously increase H-2 yield. When the gasification temperature exceeds 700 degrees C, the sharp decrease in CO2 absorption capability of CaO drastically increases the CO2 concentration and yield, which significantly decrease H-2 concentration. The appropriate temperature for the absorption-enhanced gasification process should be selected between 600 degrees C and 700 degrees C in atmospheric pressure. Increasing the water injection rate (represented as the mass ratio of steam to biomass) could also improve H-2 yield. The type of biomasses is closely associated with H-2 yield, which is closely related to the volatile content of biomass materials. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.