Oxygen permeation properties of Sr(1-x)A(x)'CoO2.5+delta (A'=Ca, Sr, Ba, and Mg) were investigated. The oxides with x less than or equal to 0.2 had hexagonal 2H-BaNiO3-type structure at room temperature. The disk membranes (1.0 mm thick) made of these oxides, when cooled from high temperature, showed significant rates of oxygen permeation down to critical temperatures characteristic of the kind of oxides, below which the rates sharply diminished. Among the oxides tested, Sr0.9Ca0.1CoO2.5+delta showed the largest rate (2.0 cm(3) at standard temperature and pressure min(-1) cm(-2)) at 900 degrees C. As indicated by the X-ray diffraction and thermogravimetry-differential thermal analysis, this oxide underwent phase transformation between the above structure (low-temperature phase) and a cubic perovskite structure (high-temperature phase), which were oxygen nonpermeable and oxygen permeable, respectively. On heating, the oxygen content (2.5 + delta) of the oxide decreased in two steps, i.e., first from 2.75 to 2.6 over the temperature range 400 to 600 degrees C without changing the structure, and then from 2.6 to 2.5 at the temperature of transformation to the high-temperature phase (about 900 degrees C). The rate of oxygen permeation of the oxide at 900 degrees C was found to depend on the membrane thickness. Based on these results, oxygen permeation and its relevance to phase transformation are discussed.