Abstract A new high-pressure form of BaCuO 2 + x has been observed in specimens synthesized at 5 GPa. It shows a fundamental structure similar to the high-temperature form of hexagonal (4H)-BaMnO 3. The compound crystallizes in an orthorhombic unit cell with the lattice parameters α = 9.827 (1) A ̊ , b = 5.6783 (7) A ̊ and c = 9.5252 (4) A ̊ and belongs to the space group P222 1. Based on both the high resolution transmission electron microscopic image and Rietveld analysis on X-ray diffraction patterns, a structural model is proposed. The calculated and observed X-ray patterns are in reasonable agreement.