Multiscale structure of two CoxAlyOz cermets was examined by a combination of XRD, SEM coupled with EDX, NMR (59Co; 27Al), and textural measurements. A strong CoAlO/CoAl cermet monolith prepared by mechanical alloying of Co and Al powders followed by hydrothermal treatment and calcination in air comprised two parts: the metal core consisting of Co and a Co–Al alloy, and the outer oxide matrix including a mixture of CoxAl3–xO4 spinel and cobalt oxides. The two parts were separated by the alumina interface protecting the metal core against the oxidation. Al2O3/CoAlO/CoAl cermet prepared by mixing the Co–Al mechanically alloyed product and pure aluminum hydroxide with subsequent hydrothermal treatment and calcination in air consisted of three main parts: (i) large cobalt-free porous alumina (ii) surrounded by CoxAl3–xO4 spinel oxides, and (iii) the inner core containing pure Co metal particles covered with the Co–Al alloy. A random distribution of metal particles in the oxide matrix provided an enhanced microwave absorption. A developed porosity ensured a high activity of the catalyst prepared from the porous cermet in the hexane dehydrogenation under microwave heating.