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 1atm shows that the use of 80wt%Na 2 CO 3 /20wt%MgO composite material successfully delayed the cooling time of the catalyst bed by 5–19min 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 10min in the gas hourly space velocity (GHSV) range of 5000–12,500h −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.