Sr 6 Co 5 O 15 , 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. Sr 6 Co 5 O 15.12 , Sr 6 Co 5 O 14.98 , Sr 6 Co 5 O 14.45 and Sr 6 Co 5 O 14.26 were prepared by a solid state reaction in air. The Sr 6 Co 5 O 15 phase is stable in the temperature range of 873–973K in air, there are structure transitions at 773K and above 1023K. With the loss of oxygen, the a-axis parameter of the Sr 6 Co 5 O 15 phase decreased from 9.4988(3)Å (Sr 6 Co 5 O 15.12 ) to 9.4390(4)Å (Sr 6 Co 5 O 14.26 ) and the c-axis parameter increased from 12.3772(3)Å (Sr 6 Co 5 O 15.12 ) to 12.5066(4)Å (Sr 6 Co 5 O 14.26 ) 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 CoCo distance between the CoO 6 trigonal prism and CoO 6 octahedron.