The X-ray diffraction (XRD) signature of the glassy phase in blast furnace slag undergoing alkaline dissolution is evaluated. The intensity signature of the glassy phase present in slag can consistently be decomposed into three underlying pseudo-Voigt (PV) peaks. It is shown that the fundamental underlying characteristics of the XRD signature of the undissolved glassy phase of slag in terms of the underlying PV peaks do not change after dissolution in an alkaline solution. The stability of the calcium ions depends on the [OH−] concentration in the solution. An intensity-based procedure is developed for quantifying the unreacted glassy phase content in alkali-activated slag. The XRD profile information of the glassy phase in raw slag is suitable for fitting the intensity profile of the dissolved glassy phase. The mass percentage of the unreacted glassy phase of slag within alkali-activated slag is validated with selective acid dissolution. A procedure for determining the degree of reaction in alkali-activated slag is established. The procedure developed here could be used to determine the activity of slag in an alkaline environment. Copyright © 2021 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.