Dual flue gas conditioning (DFGC) deals with the addition of chemical additives such as ammonia and sulfur trioxide, simultaneously, to the flue gas for increasing the ash collection efficiency of electrostatic precipitators (ESPs). Previous studies with ammonia as the conditioning agent [i.e., ammonia flue gas conditioning (AFGC)] have revealed that the collection efficiency increases drastically due to agglomeration of ash particles. However, in what way combination of sulfur trioxide and ammonia helps in increasing the efficiency of ESPs, and hence controlling emissions from the power stations, has not yet been studied in detail. With this in view, efforts were made to investigate the influence of DFGC and AFGC on the collection efficiency of ESPs and overall properties of the ash. To achieve this, fly ash samples from a thermal power station in India, where DFGC is being adopted, were collected and their physico-chemico-mineralogical and morphological properties were investigated. The results obtained were critically analyzed vis-à-vis those obtained for the fly ash samples collected from a thermal power plant where only AFGC is being adopted. The advantage of DFGC over AFGC for enhancing the overall collection efficiency of ESPs has been demonstrated in this paper. Copyright © 2009 by ASTM International.