Ultrahigh-pressure transformations in the Ce0.43Th0.57 alloy were investigated up to 253 GPa (volume compression V/V0=0.402) in a diamond anvil cell utilizing synchrotron x-ray diffraction. Unlike Ce, a first-order isostructural-fcc-to-fcc transition under pressure was not observed in this alloy. At the initiation of f-electron bonding, the fcc-to-bct transition was observed at 17 GPa. This pressure is close to the corresponding transition pressure in Ce metal (12 GPa) and is considerably lower than that in Th metal (60 GPa). The present experiments suggest that there are strong nonlinearities in transition pressure as a function of alloying content for f-electron alloys. © 1994 The American Physical Society.