A prototype 3 kW solar gasifier that operates in continuous mode to produce a mixture of hydrogen and carbon monoxide (synthesis gas) was operated at 1200 K in a high flux solar simulator with record breaking efficiency for steam gasification of cellulose. A unique aspect of the gasifier is the use of a molten alkali salt as heat transfer media and chemical catalyst. To identify an appropriate salt composition, we assess the physical solubility and chemical reactions of product and reactant species over a range of oxidizer stoichiometry. For near-stoichiometric steam oxidation, a carbonate blend of 38.6% mol Li2CO3, 29.7% Na2CO3, and 22.9% K2CO3 with 8.7% mol alkali metal hydroxide salts, including 6.1% LiOH, 1.5% KOH, and 1.1% NaOH ensures equilibrium of the salt exposed to steam gasification products and no excess production of CO2. The product is syngas with a hydrogen and carbon monoxide ratio of 1:1. Feedstock carbon conversion is 78%, and the reactor thermal efficiency is 40%. Assuming that unconverted carbon is recycled, the projected efficiency is 44%. The efficiency exceeds that previously reported for CO2 gasification due to an improved feed system.