We have investigated the variations in the crystal structure of Fe69 Ni31 Invar alloys under high pressures below 6.5 GPa and high temperatures below 1300 K using energy dispersive x-ray diffraction. The crystal structure is stabilized as a face-centered-cubic (fcc) lattice structure over the entire investigated range. However, the volume of the fcc in Fe69 Ni31 shows an abrupt decrease at 1300 K above 2 GPa and at 1100 K above 3 GPa with increasing pressure. These results show that a pressure-induced phase transition to a low-volume state occurs under high-temperature and high-pressure conditions. We consider this transition would be that from the high-spin state to the low-spin state, which has been speculated in some previous theoretical studies. © 2008 The American Physical Society.