Electrical studies of the Zn, Sn codoped bixbyite (In2O3) phase (ZITO), a promising alternative to indium-tin oxide (ITO) for transparent conductor applications, indicate that an inherent cation off-stoichiometry in favor of Sn donors versus Zn acceptors dominates the defect chemistry of this important transparent conducting oxide. This was shown by bulk phase diagram and conductivity studies, thin-film electrical/optical measurements, and photoelectron spectroscopy on both bulk and thin-film specimens. The Sn-excess character explains the persistent n-type behavior of bulk ZITO, the relative redox insensitivity of codoped compositions, the existence of "special" (optimized conductivity) compositions in phase space for pulsed laser-deposited films, and the propensity for surface chemical depletion in both bulk and thin-film specimens.