A synchrotron-radiation study has been carried out on a crystal of YBa 2 Cu 3 O 6 + x with a low oxygen content ( x = 0.15–0.20) to investigate the so-called 2√2 a 0 × √2 b 0 × c 0 phase, attributed up to now to oxygen ordering in the YBCO CuO x layers. This crystal was grown from a flux and quenched from 900°C. It is shown that neither the ordering of CuOCu monomers in the herringbone-like scheme nor any other kind of oxygen ordering in the YBCO lattice can explain the observed reflections that actually correspond to the unit cell 2√2 a 0 × √2 b 0 × c ≈ c 0 /3. A structural model is proposed that gives a very good description of the diffraction pattern; it corresponds to a parasitic phase of formula BaCu 3 O 4 that is characterized by a stacking of Cu 3 O 4 layers connected by barium atoms and that probably grows by epitaxy upon the CuO 2 planes of YBCO.