X-ray powder diffraction (XRD) in combination with the Rietveld method was used to evaluate the proportion of crystalline and amorphous (glassy) phases in fly ashes produced in pulverized-coal-combustion (PCC) power plants. The quantification of amorphous phases through the Rietveld method is most usually carried out (i) by using an internal standard or (ii) by using a poorly-crystalline phase to represent the amorphous phase in the lineshape calculations. In the present work, we explore the usefulness of an alternative method that relies on the calibration of the broad XRD signal arising from the amorphous phase. This procedure, which does not require any spiking of the samples, provides a suitable alternative to evaluate in an easy, fast and consistent manner the amount of crystalline and amorphous phases in the fly ashes.