The x‐ray induced photocurrent in the thermal SiO2layer of a Metal‐SiO2‐Si (MOS) structure consists of two components: (1) contact photoinjection and (2) bulk generation. It has been found, both theoretically and experimentally, that the relative importance of the contact photoinjection component increases with decreasing oxide thickness. For a 500‐A˚ SiO2, the contact photoinjection is approximately equal to the bulk generation over a wide range of oxide fields (105–2×106V/cm) and starts to dominate for oxides thinner than 300 A˚. The earlier internal photoemission theory developed for UV photon energies below the SiO2band gap can be used to describe the observed voltage dependence of the contact photoinjection current under x‐ray irradiation, and good agreement between the theory and the experiment has been established. Onsager’s theory of geminate recombination is shown to satisfactorily explain the field dependent generation rate in SiO2over the field range 105–2×106V/cm. These findings are important to the understanding of the ionizing radiation effects in MOS structures where carrier injection and generation play an essential role.