We have measured the O K-edge XANES, spectra of a series of standard minerals and glasses in the CaO-Al2O3-SiO2 (CAS) system. Changes observed in the spectra of simple compounds like the calcium aluminates, SiO2, AlPO4, and Al2O3, where the O atoms are surrounded by network former neighbours in four-, five-, and six-fold coordination can be used to interpret structural changes occurring in CAS glasses. In particular, changes in the coordination of Ca, as well as, changes in the Al-NBO and Al-BO distribution can be resolved. For glasses along the CaO-Al2O3 join some Ca is found to be in 9-fold coordination. However, along the (xCaO:Al2O3)-SiO2 joins (where x = 1, 1.57 and 3) the average Ca coordination decreases as the proportion of CaSiO3 increases. Furthermore, along the CaSiO3-Al2O3 join the O K-edge XANES exhibits changes consistent with AlO4 Q(2) and Q(3) species being converted to Q species with the addition of Al2O3. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.