Composite materials with alkali carbonate and magnesia have been examined for high-temperature thermal storage in solar tubular reformers. The concept of a double-walled reactor tube involves packing a molten-salt/ceramic composite material into the annular region between internal catalyst tube and exterior solar-absorber wall. In this paper, the shape and interior structure of the reactor tube are newly designed for use in solar cavity-type reformers using straight reactor tubes. Na(2)CO(3), K(2)CO(3), and Li(2)CO(3) composite materials with magnesia were tested as thermal storage media for CO(2) reforming of methane during cooling mode of the reactor tube at a laboratory scale. The efficiency of Na(2)CO(3)/MgO composite with various MgO contents was also estimated. Composite materials of Na(2)CO(3) 80-90 wt% and MgO 20-10 wt% were successfully delayed the cooling of the catalyst bed and sustained methane conversion at >90%. A solar cavity-type reformer consisting of multiple straight reactor tubes is expected to enable stable operation of the solar reforming process under fluctuating solar insolation during cloud passage. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.