The reactivity of dolomite, limestone and NiO/Dolomite toward hydrogen production during high temperature steam gasification of biomass were evaluated. NiO/Dolomite additive was prepared from impregnation of NiO on dolomite surface. Reactor with gasification bed operated at 900 degrees C and sorption/reforming bed operated at 730 degrees C at downstream was applied in this paper. Results show that co-gasification of biomass and additive increased overall gas yield and also the CO2 concentration. With dolomite in the sorption/reforming bed and none additive in gasification bed, the highest hydrogen concentration was achieved (71.8 vol.%). With 2wt.% NiO/Dolomite in the sorption/reforming bed and dolomite in the gasification bed separately, the highest hydrogen yield (510 cm(3)/g) was reached. Further increasing NiO content decreased hydrogen yield, sorption capacity and increased coke deposition. Calcination of NiO/Dolomite at 950 degrees C led to formation of NiO-MgO phase and reduced to Ni-MgO phase during gasification which increased the catalysis of the additive. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.