In the search for high-temperature thermoelectric materials, two families of novel, narrow-bandgap semiconducting antimonide oxides with the compositions RE3SbO3 and RE8Sb3-delta O8 (RE = La, Sm, Gd, Ho) have been discovered. Their synthesis was motivated by attempts to open a band gap in the semimetallic RESb binaries through a chemical fusion of RESb and corresponding insulating RE2O3. Temperatures of 1350 degrees C or higher are required to obtain these phases. Both RE3SbO3 and RE8Sb3-delta O8 adopt new monoclinic structures with the C2/m space group and feature similar REO frameworks composed of "RE4O" tetrahedral units. In both structures, the Sb atoms occupy the empty channels within the REO sublattice. High-purity bulk Sm and Ho samples were prepared and subjected to electrical resistivity measurements. Both the RE3SbO3 and RE8Sb3-delta O8 (RE = Sm, Ho) phases exhibit a semiconductor-type electrical behavior. While a small band gap in RE3SbO3 results from the separation of the valence and conduction bands, a band gap in RE8Sb3-delta O8 appears to result from the Anderson localization of electrons. The relationship among the composition, crystal structures, and electrical resistivity is analyzed using electronic structure calculations.