Fly ashes exist as a mixture of major amorphous phases and minor crystalline phases. For commercial applications, such as in concretes and for the production of zeolites, it would be desirable to be able to predict the reactivity of fly ashes. The amorphous phase dominates degradation behaviour, because glasses have 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. Despite the large quantities of fly ash produced annually by coal-burning power plants, there have been very few studies investigating the microstructure and composition of the amorphous component. In particular, there has been little research undertaken in measuring the glass transition temperature (T-g), which can be directly correlated to the chemical reactivity of the glass phase. Thirteen European fly ashes were used for the present study. Differential scanning calorimetry (DSC) was employed to determine the presence of transition temperatures and any other thermal events (exotherms or endotherms) in the glassy phase of the fly ashes. Several different but distinct behaviours were evident in the DSC traces with T-g values visible for six of the ashes. The results suggest that thermal analysis has potential as a technique for fly ash characterisation.