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Discharge-chamber sputter erosion has been investigated in two electron-bombardment mercury ion thrusters at 8-cm and 12-cm beam diameter (respectively, the SIT-8 and the SIT-12 thrusters). Sputter erosion is found to be concentrated primarily on the screen electrode and is due primarily to singly charged mercury ions, Hg + . A model of discharge-chamber sputter-erosion has been developed that predicts erosion rates that are in good agreement with observed rates. A three fold reduction in the rate of total mass erosion has been achieved in both the SIT-8 and SIT-12 thrusters by biasing the thruster shell (including screen electrode) at keeper potential rather than at cathode potential.