Abstract: The effectiveness of electrostatic precipitators, and particularly existing electrostatic precipitator installations, is enhanced by introducing into the gas stream in advance of the precipitator a spark-inhibiting additive, and then increasing the voltage applied to the precipitator over that which otherwise could be applied. The spark-inhibiting additive is preferably an ammonium salt introduced into the gas stream at a relatively low temperature and preferably in combination with the introduction into the gas stream at a relatively higher temperature of known conditioning additives from the group consisting of sodium bisulphate, ammonium bisulphate, ammonium sulphate and mixtures thereof.