The injection of powdered sorbents, such as activated carbon, for mercury emissions control at coal-fired power plants has primarily taken place upstream of electrostatic precipitators (ESPs), which far outnumber baghouses in the U.S. Although full-scale sorbent injection tests have demonstrated varying degrees of mercury removal efficiency, the actual behavior of powdered activated carbon (PAC) within an ESP has not been well-established, particularly as this behavior relates to adsorbing gas-phase mercury. In the present experimental investigation, results obtained in a lab-scale ESP indicate that the electrical properties of PAC may cause its collection in a full-scale ESP to be significantly different from that of the native fly ash. There appears to be potential for significant collection of PAC on the discharge electrode wires of an ESP. Because these wires are typically not rapped as frequently as collection electrodes in an ESP, over time, such behavior could potentially create a series of cylindrical PAC structures that contribute non-negligibly to the total mercury removal efficiency within the ESP. To test this potential, the present investigation also presents results from a mass transfer model, in which the PAC-coated discharge electrodes of an ESP are represented as a row of cylindrical mercury sinks, whose radii and ultimate adsorption capacity for mercury vary in time. Results from the mass transfer model show that, after extended periods of PAC injection, the PAC-covered discharge electrodes would continue to adsorb mercury from the flue gas after injection ceases. Over time, the collected PAC becomes saturated, producing a slow rise in the measured mercury concentration at the ESP outlet. This trend agrees with that occasionally observed after full-scale sorbent injection tests and suggests a secondary mechanism for mercury adsorption within ESPs. Such anomalous behavior may require a separate evaluation to assess its impact on ESP operations and maintenance.