The crystal structures and characterizations of Ba(4)EuM(3)O(12) (M = Ru and Ir) are reported. They crystallize in a monoclinic 12-l-perovskite-type structure with space group C2/m. The M(3)O(12) trimers and EuO(6) octahedra are alternately linked by corner-sharing and form the perovskite-type structure with 12 layers. The (151)Eu Mossbauer and X-ray photoelectron spectra reveal that the Eu and M ions are in the trivalent and mixed valence (+4.33) oxidation states, respectively. Their electrical conductivities show semiconducting behavior with an activation energy of approximately 0.2 eV above room temperature and demonstrate two-dimensional Mott variable range hopping behavior at low temperatures. The magnetic susceptibility and specific heat measurements indicate that an antiferromagnetic ordering occurs at 4 K, which is due to the magnetic interaction between Ru(3)O(12) trimers. On the other hand, the Ir(3)O(12) trimer is found to be nonmagnetic.