The onset of f-bonding as a result of electron transfer under pressure has been investigated experimentally by interpreting structural distortions which accompany this phenomenon in light actinide and lanthanide metals. Structural studies were carried out on the light actinide metal, thorium (Th), below 100 GPa and also in the ultrahigh pressure regime to 300 GPa by energy dispersive X-ray diffraction techniques using synchrotron radiation sources. Based on the forward and reverse transformation data, the FCC to BCT equilibrium pressure for Th has been determined to be 60 ± 4 GPa at 300 K. Detailed comparisons are made between the phase transformations of Th and those of the light lanthanide metal, cerium (Ce). The intermediate body centered monoclinic phase, reported to exist between the FCC and the BCC phases in Ce, was not observed for Th. The possible scaling relations between the phase diagrams of Ce and Th are discussed. © 1993.