Ir-substituted barium hexaferrites (BaIrxFe12-xO19, x <= 0.6) were prepared using the carbonates route and investigated for high-concentration N2O decomposition. It was found that BaIrxFe12-xO19 offered a better dispersion of iridium by incorporating Ir into the hexaferrite structure. The crystallographic sites of Ir in BaIrxFe12-xO19 were identified through the analysis of Fe occupancy in the matrix by sensitive Fe-57 Mossbauer spectroscopy. Framework iridium preferentially occupied the octahedral sites in the order of 2a, 12k, and 4f(2) as increasing x value. When x = 0.1 and 0.4, the substitution of Ir for Fe3+ occurred in the 2a and 12k sites in the rigid spinel block with a comparable N2O decomposition activity, while this substitution in the 4f(2) sites in the mirror plane at x = 0.6 resulted in a remarkable enhancement of activity. Ir ions in the octahedral 4f(2) sites in the loosely packed mirror plane were highly active for N2O decomposition. (C) 2011 Elsevier B.V. All rights reserved.