A large quantity of fly ash is being used in various civil Engineering applications all over the world. Proper characterization of fly ash is essential to utilize the fly ash to its full reactive potential. results from an experimental program using multiple characterization techniques on samples of fly ash collected from different sources located in the Southern part of India are presented. the composition of fly ash is determined using X-ray flouresence (XrF) spectroscopy and the total reactivity of fly ash is established using an X-ray diffraction (Xrd) spectorscopy. It is shown that the fly ash composition and reactivity varies with the source. the reactive silica and the reactive alumina contents of fly ash are determined using combined Xrd and XrF techniques. the reactive silica content determined using the combined X-ray based techniques compared favorably with the dissolution procedure given in IS 3812 (part-1). Significant proportions of the silica and alumina in the fly ash are present in the non-reactive crystalline forms associated with quartz and mullite. the total contents of silica and alumina obtained from the oxide composition do not provide any indication of the reactive silica and the reactive alumina contents present in the amorphous phase of fly ash. there is no clear relationship between the total silica and the reactive silica in the fly ash. Similarly, there is no correlation between the total alumina and the reactive alumina present in the amorphous form. An inverse linear relationship is established between the Mullite content in the fly ash and the iron content in its amorphous phase. there is a significant Fe content in the amorphous phase of the fly ash, which is identified with specific textured spherical particles in the size range from 5 to 8 microns. All the fly ashes have a very low calcium content, which is predominantly present in its amorphous glassy phase. © 2018 Associated Cement Companies Ltd.. All rights reserved.