The structural changes of a flint aggregate attacked by alkali-silica reaction (ASR) were investigated using an environmental scanning electron microscope (ESEM) equipped with a X-ray microanalysis system. Compared to the conventional SEM, the ESEM enables the direct imaging of the samples in their natural state. The results obtained are compared to those acquired using X-ray diffraction (XRD). It appears that when calcium is present in the aggregate, the attack takes place from outside and progresses towards the inside of the aggregate. It has also been shown that calcium has an influence in slowing down the reaction by preventing the fast degradation of the aggregate. In addition, it supports the formation of new internal phases in the aggregate. The ESEM and XRD data were found to be consistent with each other. (C) 2004 Kluwer Academic Publishers.