The paper gives results of an evaluation of pilot electrostatic precipitator (ESP) performance with elevated loadings from the advanced silicate (ADVACATE) sorbent injection process. Measurements were made of a calcium silicate sorbent injected into a duct upstream of an ESP. The concentration of ADVACATE sorbent submicron particles (=/< 1 micrometer) and projected ESP emissions tended to peak and began to decrease when the overall particulate matter addition rate to the gas stream approached and then exceeded 12 g/Nm3. The submicron fly ash, subjected to the same duct injection, increased linearly with increased injection rates from 3 to 24 g/Nm3. A possible explanation is in-duct agglomeration of fines by the coarse particles, similar to observations reported on cyclone performance evaluations. The duct, flue gas, and sorbent characteristics that affect agglomeration tendencies probably play a major role in the observations presented. Most of the ADVACATE material settled out of the gas stream. Measurements of the gas-suspended residual particulate matter were used to model expected ESP performance. The encouraging results of the modeling suggest that collection of reacted ADVACATE sorbent in a ESP is manageable.