Combined solid state phase diagram studies and physical property measurements of the various n-type transparent conducting oxide (TCO) phases in the CdO-In 2 O 3 -SnO 2 system have been carried out. The 1175°C (air) subsolidus phase diagram has been established, including solid solution limits for binary and ternary compositions. From these limits and electrical property measurements vs. doping and degree of reduction, the prevailing defect mechanisms can be deduced. In addition to intrinsic (native) defects (e.g. oxygen vacancies) and extrinsic donor-doping of the end member compounds (e.g. Sn 1n in In 2 O 3 ), ternary solid solutions exhibit both isovalent doping (e.g. [Cd' In ] = [Sn In ] in bixbyite, spinel) and donor-to-acceptor imbalance (e.g. [Sn In ] > [Cd' In ] in bixbyite, spinel). Aliovalent doping can also lead to the formation of point defect associates, as in Sn-doped In 2 O 3 (ITO), as confirmed by combined Rietveld analyses of X-ray and neutron diffraction data. Cation exchange between sublattices in the spinel phase plays an important role in determining phase stability and band structure. The physical properties of the TCO phases in the CdO-In 2 O 3 -SnO 2 system are presented for both bulk ceramics and thin films.