Double perovskites containing Ir6+/Ir5+ with formula Sr2MIrO6 (M = Ni, Zn) have been synthesized under high oxygen pressure conditions. Their crystal structures have been studied by X-ray and neutron powder diffraction at room temperature (RT) and 2 K. At RT, these oxides crystallize in the monoclinic space group P2(t)/n with unit-cell parameters a approximate to root 2a(0), b approximate to root 2a(0), and c approximate to 2a(0), and beta approximate to 90 degrees. The thermal evolution of the structure of the Ni-containing compound shows the presence of two phase transition in the 373-673 K interval following the sequence P2(1)/n -> I4/m -> Fm (3) over barm. These materials have also been characterized by magnetic measurements, suggesting the onset of antiferromagnetic interactions at T-N = 58 and 46 K, for M = Ni, Zn, respectively. X-ray absorption spectroscopy sheds light on the oxidation states of M and Ir ions within these double perovskites.