Diffusionless phase transitions of Fe-Ni alloy with 30 wt % Ni (Fe30Ni) were studied at high temperature and pressure by synchrotron radiation. Several isothermal runs ranging from 25¿C to 600¿C were carried out at up to 20--60 GPa to determine the phase relationships and elastic properties of the alloy. The ruby fluorescence method was used for pressure measurements at room temperature. The compressibility of the fcc phase of the alloy, determined up to 48 GPa, has a bulk modulus of 160¿15GPa at room temperature assuming K0'=4. The average linear thermal expansion coefficient of the fcc phase of Fe30Ni was estimated to be 10.2¿1.5¿10-6 K-1 between 25¿C and 600¿C. The pressure at high temperature was calculated using the equation of state of the fcc phase assuming that the compressibility of the alloy at high temperature remains the same as at 25¿C. The Fe30Ni alloy has both fcc and bcc structures at ambient conditions. However, the bcc phase disappeared (bcc-out) after pressure was applied to the sample at various temperatures. The bcc-out transition boundary has a negative slope of -55¿¿5¿C/GPa. The fcc phase transformed to a hexagonal close-packed phase at high pressure (hcp-in). The slope of the phase boundary of hcp-in was determined to be 25¿5¿C/GPa. This result, combined with our previous investigation (Huang et al., 1988), indicates that the slope of the fcc/hcp phase boundary in Fe-Ni alloy remains constant with an increase of Ni content up to 30 wt %. ¿ American Geophysical Union 1992