The amorphous phase of fly ash dominates degradation behaviour because glass has a higher potential energy than the equivalent crystal structure and the variation of bond angles and distances in a glass make the bond breakage easier. It would be advantageous to predict the presence and subsequent degradability of glass on the basis of the solid-state chemistry of the fly ash. To this end, and inorganic polymer model was applied to a selection of European fly ashes to determine the value known as cross-link density (CLD). A cross-link density value of less than two implies that the material is amorphous in nature and the lower the CLD below two, the greater the reactivity and solubility of the glass. Applying this model may facilitate the selection of the most suitable fly ash for a particular recycling application where glass reactivity or dissolution rates are important. To check the applicability of the model to the glass phase of fly ashes, CLD calculations have been performed by removing the contribution to the ash composition from the known crystal phases. The model would be then expected to give a maximum CLD value of two for all the materials. While this approach has been applied successfully to synthetic glasses and glass-ceramics in the past, only very limited applicability has been found with fly ashes. This is believed to be due to the inherent heterogeneity of the glass phase in fly ash.