A device for the thermal separation of water into hydrogen and oxygen, including a closed reaction chamber (1) containing water and, in said reaction chamber: —a heating system including one or several heat source elements (4,11), —one or several membranes (3), essentially impermeable to gas, to permit the selective passage of oxygen, —one or several membranes (2), essentially impermeable to gas, to permit the selective passage of hydrogen and —a mechanism (5) to permit the passage of water into said reaction chamber. According to the invention, —said heat source(s) (4, 11) is(are) placed in the water inside said reaction chamber (1), and, —said selective membranes (3) for oxygen are placed in said zones at high temperatures, —said selective membranes (2) for hydrogen are placed in said zones at lower temperatures. Preferably, the heating system is comprised of one or several concentrators (8, 9) of solar rays focusing the rays toward the inside of the reactor.