The ternary antimonide CeIrSb absorbs hydrogen under moderate temperature and pressure conditions (4 MPa and 573 K), leading to the hydride CeIrSbH_0.8. The crystal structures of both compounds have been investigated by X-ray diffraction on powders and single crystals: TiNiSi type, space group <i>Pnma</i>, <i>a</i>=735.07(7), <i>b</i>=456.93(4), <i>c</i>=792.8(1) pm, <i>R</i>1/w<i>R</i>2=0.0206/0.0395, 601 <i>F</i>² values for CeIrSb and <i>a</i>=728.16(14), <i>b</i>=460.35(6), <i>c</i>=825.87(2) pm, <i>R</i>1/w<i>R</i>2=0.0322/0.0735, 528 <i>F</i>² values for CeIrSbH_0.8 with 20 variables per refinement. Hydrogenation induces both an increase of the cell volume <i>V</i> (+4%) and a strongly anisotropic expansion of the unit cell with a maximum of 4.3% in the c direction, leading to a significant increase of the Ce–Ir and Ce–Ce distances in this direction. The H-insertion into CeIrSb leads to a magnetic transition from intermediate valence to antiferromagnetic behavior (<i>T</i>_N=7.0 K) evidenced by magnetization, electrical resistivity and specific heat measurements. This transition can be explained on the basis of the Doniach diagram considering the <i>J_cf</i> interaction between the 4<i>f</i>(Ce) and conduction electrons.Read less <