Hydrogen (H(2)) and other gases (CO(2), CO, CH(4), H(2)O) produced during the pyrolysis of cellulose, xylan, lignin and pine (Pinus radiata), with and without added calcium oxide (CaO), were studied using thermogravimetry-mass spectrometry (TG-MS) and thermodynamic modeling. CaO improved the H(2) yield from all feedstocks, and had the most significant effect on xylan. The weight loss of and gas evolution from the feedstocks were measured over the temperature range 150-950 degrees C in order to investigate the principle mechanism(s) of H(2) formation. Without added CaO, little H(2) was produced during primary pyrolysis; rather, most H(2) was generated from tar-cracking, reforming, and char-decomposition reactions at higher temperatures. When CaO was added, significant H(2) was produced during primary pyrolysis, as the water-gas shift reaction was driven toward H(2) formation. CaO also increased the formation of H(2) from reforming and char gasification reactions. Finally, CaO increased the extent of tar cracking and char decomposition, and lowered their onset temperatures. The production of H(2) from pine over the course of pyrolysis could be modeled by summing the H(2) evolutions from the separate biomass components in relevant proportions. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.