Synchrotron X ray diffraction analysis of a nickel superalloy subjected to barothermal treatment (BTT) at 1235, 1260, and 1320°C is performed using a high power synchrotron radiation source. The alloy is found to contain two types of γ' phase and two types of γ solid solution, which are likely to be located in dendrite arms and the interdendritic space, respectively. The averaged lattice misfits of the γ' phases and the γ solid solutions change with the BTT temperature at a constant pressure and exposure time, which substan tially affects the high temperature characteristics of the alloy.