A method is provided for enhancing the efficiency of fly ash collection in an electrostatic precipitator by lowering the resistivity of the fly ash particles in the flue gas, comprising the steps of introducing an aqueous solution of ammonium sulfate or ammonium bisulfate into a slipstream of hot flue gas or hot combustion air in a chamber external to the main flue gas duct wherein thermal decomposition is effected, removing the NH.sub.3 decomposition product by catalytic oxidation to increase the effectiveness of the SO.sub.3 decomposition product, and then distributing the SO.sub.3 decomposition product into the main flue gas stream exiting from the air preheater at a point upstream from the electrostatic precipitator.