Abstract The formation of superconducting oxide (SCO) phases in the homologous series Yb n Ba 2 n Cu 3 n+1 O x ( n=1,2,∞) in 1 atm of O 2 was investigated by high-resolution and conventional transmission electron microscopy, energy dispersive X-ray spectroscopy, and powder X-ray diffraction. The sequence of SCO phase formation at 847°C was YbBa 2Cu 3O 7−δ ( n=∞), YbBa 2Cu 4O 8−δ ( n=1), and finally Yb 2Ba 4Cu 7O 15−δ ( n=2). YbBa 2Cu 4O 8−δ formed at the expense of YbBa 2Cu 3O 7−δ, whereas, Yb 2Ba 4Cu 7O 15− x formed at the expense of both YbBa 2Cu 3O 7−δ and YbBa 2Cu 4O 8−δ. The formation of Yb n Ba 2 n Cu 3 n+1 O x ( n=1,2) occurs by intercalation of additional Cu 2+ and O 2− along a-b planes in the parent YbBa2Cu 3O 7−δ structure creating double CuO layers every n YbBa 2Cu 3O 7−δ “building blocks”. The decomposition of Ba 2Cu 3O 5 provides the Cu 2+ and O 2− necessary for these phase transformations.