Synchrotron radiation micro computed tomography (SR Micro CT) and micro X-ray diffraction (SR Micro XRD) were used to investigate the deleterious effects of alkali-silica reaction (ASR) in mortar bars. The samples were prepared by mixing ordinary Portland cement and chert, the latter consists of quartz crystals known to be potentially alkali-silica reactive; then they are aged in a NaOH solution at 80 A degrees C according to RILEM AAR-2 for ASR to occur. A characterization of the microstructural features (cracks, voids due to dissolution, aggregate detachment) due to ASR was performed by SR Micro CT and a detailed mineralogical characterization of the weathering layer growing at the cement paste-aggregate interface was conducted by SR Micro XRD. When ASR occurs, we observe the dissolution of the quartz belonging to aggregate followed by the precipitation of new crystals of quartz. On the other hand, when ASR aging increases the quartz dissolution is almost complete and a halo diffuse scattering dominates the XRD patterns. Furthermore, the ASR generated a widespread microcracking associated with irregular voids due to aggregate dissolution and a general detachment at the cement paste-aggregate boundary is observed.