Three samples from the ceramic melt, the lower crust, and the lower plenum of the previously molten part of the Three Mile Island Unit 2 core have been analyzed by X-ray diffraction to determine the compounds formed as a result of the accident in 1979. Supporting analyses were performed by gamma spectroscopy and particle-induced X-ray emission (PIXE) to provide information on radioactive nuclide content at trace levels and on elements with Z> Mat more significant levels. The analyses show the presence of the following major phases: (a) an inhomogenous solid solution based on UO2, probably containing zirconium as a substituent for uranium and with additional oxygen giving a superstoichiometric composition; (b) ZrO2 in the baddeleyite modification, which is stable below 1200 K, and tetragonal ZrO2, which is normally stable between 1200 and 1600 K; and (c) nickel, chromium ferrite [(Ni,Fe)(Fe,Cr)2O4]. Lattice parameter measurements indicate that both forms of ZrO2 contain UO2 in solid solution and the parameter of the ferrite phase is consistent with substitution of aluminum for part of the chromium and iron content. The PIXE measurements show that the nickel content of the ferrite is low. The distribution of these phases in the samples has been studied by making quantitative measurements on diffraction patterns from a total of 34 X-ray specimens. Differences between the three samples are discussed in terms of the equilibrium diagram of the UO2-ZrO2 system. The sample from the lower plenum has evidently been subjected to rapid cooling. The temperature history of the sample from the lower crust has been such that cooling was slow enough to bring about nearly complete equilibrium of the phases. The sample from the ceramic melt represents an intermediate case with simultaneous heating at one surface and cooling at the other.