In the search for high-temperature thermoelectric materials, two families of novel, narrow-band-gap semiconducting antimonide oxides with the compositions RE(3)SbO(3) and RE(8)Sb(3-delta)O(8) (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 RE(2)O(3). Temperatures of 1350 degrees C or higher are required to obtain these phases. Both RE(3)SbO(3) and RE(8)Sb(3-delta)O(8) adopt new monoclinic structures with the C2/m space group and feature similar REO frameworks composed of "RE(4)O" 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 RE(3)SbO(3) and RE(8)Sb(3-delta)O(8) (RE = Sm, Ho) phases exhibit a semiconductor-type electrical behavior. While a small band gap in RE(3)SbO(3) results from the separation of the valence and conduction bands, a band gap in RE(8)Sb(3-delta)O(8) 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.