The intensity of solar soft x-rays (50-• -230A) as computed by Elwert is found to be sufficient to cause the ionization of the ionospheric E layer. On the other hand, preionization of molecular oxygen by ultraviolet chromospheric radiation (X<1020A, Nicolet) could perhaps also be admitted, if it could be accepted that the dissociation of atmospheric oxygen occur in a high transition layer of considerable thickness (100-150 km). The first process is probably predominant.