Deformation of tonalite and adamellite from the western Sierra Nevada, California, and southeastern Australia has produced mylonite zones that show a broad range of microstructural development, varying from strongly banded and/or foliated to locally non-foliated. Accompanying these fabrics are new mineral assemblages, commonly rich in quartz and epidote. Whereas the banded foliated varieties appear to develop from progressive deformation of the granitic host, they also occur in close proximity (cm-m) to domains of non-banded and/or weakly to non-foliated fabrics of similar mineralogy, suggesting a complex timing and partitioning of deformation, fluid flow and metamorphism in the mylonite zone. Whole-rock major and trace element analyses of the tonalites show large enrichments of SiO2, CaO, Fe2O3, total Fe and Sr, and strong depletions in Na2O, K2O, FeO, MgO, Ba and Rb with increasing stages of deformation, although substantial chemical change appears to occur under static conditions locally. Chemical changes in the adamellite are much less dramatic. All suites show systematic decrease in σ18O and convergence of oxygen isotopic compositions of whole-rock and quartz with increasing deformation. Minimum fluid/rock is estimated to lie between 0.1 and 10, increasing with intensity of deformation and depending on host composition. Isotopically exchanged seawater or connate water are the most plausible fluids. Chemical data from a number of mylonite zones in various orogenic belts show widely varying behavior (gain/loss) of major and minor elements. Eleven factors control which (and how much) elements are gained/lost during mylonitization of the granitoids, the most influential of which are primary composition, mineralogy and primary texture of the host, as well as several aspects of fluid interaction active during deformation. These influences may be a function of local rather than regional geological conditions.