The perovskite PbCrO 3 is an antiferromagnetic insulator. However, the fundamental interactions leading to the insulating state in this single-valent perovskite are unclear. Moreover, the origin of the unprecedented volume drop observed at a modest pressure of P = 1.6 GPa remains an outstanding problem. We report a variety of in situ pressure measurements including electron transport properties, X-ray absorption spectrum, and crystal structure study by X-ray and neutron diffraction. These studies reveal key information leading to the elucidation of the physics behind the insulating state and the pressure-induced transition. We argue that a charge disproportionation 3Cr 4+ → 2Cr 3+ + Cr 6+ in association with the 6s-p hybridization on the Pb 2+ is responsible for the insulating ground state of PbCrO 3 at ambient pressure and the charge disproportionation phase is suppressed under pressure to give rise to a metallic phase at high pressure. The model is well supported by density function theory plus the correlation energy U (DFT+U) calculations. Fermi energy in a partially filled band system to give rise to a Mott insulator without changing the translation symmetry (1). However, how to justify the insulating ground state in the cubic perovskite PbCrO 3 with a 2/3-filled band remains controversial (2-11). An unphysically large U needs to be used in the density function theory (DFT) calculation (10) to open a gap, indicating that electron-electron correlations alone is insufficient to account for the insulator phase. More surprisingly, the structure undergoes a first-order transition at P = 1.6 GPa to another cubic phase with an extremely large volume drop (6). To clarify the fundamental interactions leading to the cubic insulating state and whether the pressure-induced volume collapse is accompanied with an insulator-metal transition, we carried out a suite of high-pressure experiments including structural characterization, measurements of resistivity, and X-ray absorption near edge structure (XANES) under high pressure and performed DFT with Hubbard U correction (DFT+U) calculations. Detailed information about the experiments, the DFT calculation, and the simulation for XANES is provided in SI Text.The PbCrO 3 perovskite was known to be stabilized under high pressure and high temperature (HPHT) in the 1960s (2). Structural studies by X-ray and neutron diffraction revealed that it crystallizes as a cubic Pm-3m perovskite with a lattice constant of a 0 ∼ 4.00 Å and exhibits a type G antiferromagnetic (AFM) order below T N = 240 K in contrast to the type C AFM order of CaCrO 3 below T N = 90 K and that of the tetragonal phase of SrCrO 3 . The magnetic moment on Cr 4+ as refined from neutron diffraction is 1.9 μ B , which is very close to the spin-only moment of 2 μ B expected for localized d 2 electrons. In comparison with other Cr 4+ -containing perovskites, ACrO 3 (A = Ca, Sr) (9, 12), PbCrO 3 exhibits peculiar structural and physical properties. The unit-cell volume V 0 of PbCrO 3 is significantly larger than expec...