Hydrolytic weakening has been suggested as a major process facilitating strain localization, in line with many studies that found a positive correlation between water content and intensity of deformation. We examine the role of water in the unusually thick ultramylonite of the El Pichao shear zone, northwestern Argentina. We used Fourier transform infrared spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly deformed rocks. Quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks, contrary to findings in previous studies. We propose that the kilometer-thick ultramylonite formed in three stages: (1) localized deformation and recrystallization caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite; (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening; and (3) water migrated to less deformed rocks causing hydrolytic weakening, repeating the cycle and widening the ultramylonite.