Sr6Co5O15, a member of the (A(3)CO(2)O(6)) m (A(3)Co(3)O(9))(n) [A = Ca, Sr, Ba] series, showed oxygen nonstoichiometry. Sr6Co5O15.12, Sr6Co5O14.98, Sr6Co5O14.45 and Sr6Co5O14.26 were prepared by a solid state reaction in air. The Sr6Co5O15 phase is stable in the temperature range of 873-973 K in air, there are structure transitions at 773 K and above 1023 K. With the loss of oxygen, the a-axis parameter of the Sr6Co5O15 phase decreased from 9.4988(3) angstrom (Sr6Co5O15.12) to 9.4390(4) angstrom (Sr6Co5O14.26) and the c-axis parameter increased from 12.3772(3) angstrom (Sr6Co5O15.12) to 12.5066(4) angstrom (Sr6Co5O14.26) with decreasing oxygen content. The Rietveld analysis of the powder X-ray diffraction data suggested that the increase in the c-axis was mainly due to the increase in the Co-Co distance between the CoO6 trigonal prism and CoO6 octahedron. (c) 2005 Elsevier Ltd. All rights reserved.