SiO2-supported molten alkaline metal oxides (A-V-O/SiO2) were studied as SO3 decomposition catalysts for solar thermochemical water splitting. Their catalytic activities at moderate temperatures (<= 600 degrees C), which were superior to those of Cu-V-O/SiO2 catalysts, were dependent on A, exhibiting the following sequence: Cs > Rb > K > Na. These activities increased with the A/V ratio. This result is in accordance with the basicity, which favors the adsorption of SO3 to form sulfate. Another important effect of A is to form molten liquid phases, which dissolve the sulfate and facilitate its decomposition to SO2/O-2. However, the molten phase with high A/V ratios led to the collapse of the porous SiO2 structure by a corrosion effect. Consequently, the highest catalytic activity was achieved at a composition of A/V RI approximate to 1.0 for A = K and Cs. The long-term stability test of K-V-O/SiO2 at 550 degrees C demonstrated no indication of noticeable deactivation during the first 100 h, whereas 20% deactivation occurred during the following 400 h. The deactivation mechanism involves the vaporization loss of active components from the molten phase, which is accelerated in the presence of SO3.