A novel biomass conversion pathway that integrates carbon capture and storage schemes has been investigated using hemicellulose as the biomass feedstock. The alkaline thermal treatment (ATT) produces high purity H2 from biomass mixed with hydroxide and it can be considered as a bio-energy with carbon capture and storage (BECCS) technology, since the carbon in biomass becomes solid carbonates during the ATT reaction. In this study, in the presence of Ni catalyst, Group 1 and 2 hydroxides are employed to produce as high as 60 mmol H2 (96.8% purity) from 1 gram of hemicellulose (xylan) feedstock with in-situ carbon capture at mild conditions (i.e., atmospheric pressure and relatively low temperatures ≤ 773 K). Reaction pathways of hydrogen production in the ATT of hemicellulose are proposed via the investigation of the roles of each hydroxide and catalyst in the H2 formation from hemicellulose. Along with the previous studies on glucose and cellulose, this research lays a foundation for future development of H2 production from real biomass with carbon-neutral or possibly negative potential.