The composite materials of molten alkali-carbonate/MgO-ceramics are examined as thermal storage media in a tubular reformer using a double-walled reactor tube of a laboratory scale. The concept of a double-walled reformer tube is proposed as a solar tubular reformer and involves packing a molten salt/ceramic composite material in the annular region between the internal catalyst tube and the exterior solar absorber wall. The composite materials of Na(2)CO(3), K(2)CO(3), and Li(2)CO(3) with magnesia are tested as thermal storage media. The reforming performances of the composite materials are tested in the cooling mode of the double-walled reactor tube. The experimental result obtained under feed gas mixture of CH(4)/CO(2) = 1:3 at 1 atm shows that the use of 80 wt%Na(2)CO(3)/20 wt%MgO composite material successfully delayed the cooling time of the catalyst bed by 5-19 min in comparison to the case without a composite material. In addition, the Li(2)CO(3)/MgO and Na(2)CO(3)/MgO composite materials relatively revealed good performances: they prolonged the cooling time by over 10 min in the gas hourly space velocity (GHSV) range of 5000-12,500 h(-1). The application of the reactor tubes to solar tubular reformers is expected to realize stable operation of the solar reforming process under fluctuating insolation during cloud passage. (C) 2008 Elsevier Ltd. All rights reserved.