A process and apparatus for the continuous dissociation of water into hydrogen and oxygen by a thermolytic reaction in contact with a surface (9a) within the reactor immersed in a body (19) of liquid water and heated by an energy input, in particular by focusing solar radiation through a transparent wall (4), so as to maintain the thermolytic temperature at the core of the body of water, whereby a state of boiling is maintained in the immediate vicinity of the surface and generates a gas film near it; the body of water is provided in sufficient quantity to preserve in liquid form the water around that gas film, and within this film the vaporized water molecules are decomposed and in-situ chilling of the decomposition products is assured by the neighboring liquid water.