Synchrotron x-ray diffraction analytical techniques have been used to investigate the structure of oxide films formed on Type 316L (UNS S31603) austenitic stainless steel in simulated pressurized water reactor primary water. An in situ technique for investigating the layer structures of oxide films has been developed using this measurement method. The observed layer structures of the oxide films changed depending on the dissolved hydrogen concentration (DH) in PWR primary water. In two cases, where DH = 5 cm(3)/kg (H2O) or 30 cm(3)/kg (H2O), a (NixFe(1-x))Fe2O4-type spinel oxide was observed as the outer oxide, and a FeCr2O4-type spinel oxide was detected as the thin inner oxide. When DH = 30 cm(3)/kg (H2O), the Fe:Ni ratio in the (NixFe(1-x))Fe2O4 outer spinel oxide was much larger than when DH = 5 cm(3)/kg (H2O). In addition, sequential in situ measurements when the hydrogen concentration varied from 5 cm(3)/kg (H2O) to 30 cm(3)/kg (H2O) also demonstrated that the oxide layer structure seemed to adjust its characteristic composition as a function of the DH. The oxide layer structure could also be reversed to that of the initial state with DH = 5 cm(3)/kg (H2O) when the DH was switched back from 30 cm(3)/kg (H2O) to 5 cm(3)/kg (H2O).