The development of a process of hydrogen production by solar thermal water splitting (HSTWS) presents a formidable technological task. The process has, however, great potential from the thermodynamic point of view and, when combined with fuel cell technology, it can lead to efficient conversion of solar energy to power. In the process under development at the Weizmann Institute of Science, water vapor is partially dissociated in a solar reactor at temperatures approaching 2500 K. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane. The paper describes the problems encountered during the development of the HSTWS process. The following topics are discussed in some detail: (a) achievement of very high solar hydrogen reactor temperatures by secondary concentration of solar energy; (b) materials problems encountered in the manufacture of the solar reactor; (c) development of special porous ceramic membranes that resist clogging by sintering at very high temperatures.