In-based transparent conducting oxides (TCOs) share the prevailing defect structure of indium-tin oxide (ITO), i.e. electrons, isolated Sn-In donors, and neutral associates, believed to be (2Sn(In)O(i)")(x). The present work reviews the state of the literature, presents calculated Brouwer diagrams vs. oxygen partial pressure and vs. dopant concentration, and reports intermediate temperature electrical property data (thermopower, conductivity) vs. pO(2) and Sn concentration for three systems - polycrystalline hulk ITO, nanocrystalline ITO, and the recently reported ternary cation TCO, Ga3-xIn5+xSn2O16. The influence of non-reduceable tin-oxygen complexes at high doping levels is identified for ITO. Ramifications for in-based TCO properties are discussed.