In our efforts to tune the charge transport properties of the recently discovered RE3SbO3 phases (RE is a rare earth), we have prepared mixed ((REREII)-R-I)(3)SbO3 phases (REI = La, Ce; REII = Dy, Ho) via high-temperature reactions at 1550 degrees C or greater. In contrast to monoclinic RE3SbO3, the new phases adopt the P4(2)/mnm symmetry but have a structural framework similar to that of RE3SbO3. The formation of the tetragonal ((REREII)-R-I)(3)SbO3 phases is driven by the ordering of the large and small RE atoms on different atomic sites. The La1.5Dy1.5SbO3, La1.5Ho1.5SbO3, and Ce1.5Ho1.5SbO3 samples were subjected to elemental microprobe analysis to verify their compositions and to electrical resistivity measurements their thermoelectric potential. The electrical resistivity data indicate the presence of a band gap, which is supported by electronic structure calculations.