It is shown that the deformation state of a granitic rock has a profound impact on the long-term stability of concrete, if used as aggregate due to enhanced susceptibility to the alkali-silica reaction. An investigation of the microstructure of granitic rocks from the Santa Rosa mylonite zone in southern California with transmission electron microscopy and neutron diffraction revealed that, as these rocks become progressively deformed from granite to mylonite and phyllonite, accompanied by grain size reduction, the dislocation density in quartz (investigated with TEM) increases and preferred orientation of biotite (determined by neutron diffraction) becomes stronger. While the contribution of dislocations to the bulk energy increase of quartz is low, dislocations provide favorable sites for dissolution and precipitation to occur. A comparison with ASTM C 1260 expansion tests of these same samples indicates that expansion increases with the dislocation density.