While conductive carrier particles are a necessary component of a CMB developer, other developer components can also strongly affect the overall conductive performance. For example, since toner particles are insulative, the conductivity of a toned CMB developer will always be lower than that of the base carrier particles; in general, the conductivity of toned carrier particles (i.e., a conductive developer) will be an exponentially declining function of the toner concentration, leading to an eventual insulative failure state at high toner concentrations. A second important property of conductive developers is the so-called breakdown voltage, i.e., the voltage at which the developer conducts a large current (e.g., 0.1 mA). The breakdown voltage value is typically an increasing function of the toner concentration; ideally, a conductive developer will combine a high level of conductivity with a high breakdown voltage value. Film-forming external toner additives tend to reduce the influence of toner concentration on developer conductivity. Indeed, during extended use, developer conductivity can be maintained or even increased through the use of film-forming additives. However, such additives may also reduce the breakdown voltage of an aged conductive developer, and for such cases, the failure mode will be electrical shorting if a high bias is applied across the development brush. This report illustrates the varying effects of toner external additives on the aging performance of conductive developers, using experimental data taken on a range of model conductive developers based on a single conductive carrier.