Water produces changes in the properties of human keratin fibers, such as hair and nails, and therefore plays an important role in their cosmetic performance. Reactive cosmetic treatments of hair and nails often impair fiber structure, resulting in an adverse effect on water absorption. The moisture absorption/desorption isotherm curves for untreated hair and nails and the kinetics of these processes are studied in this work. The effects of different chemical cosmetic treatments on hair and nail water absorption are also evaluated. The isotherms for these human keratinized tissues behaved as expected, with a characteristic hysteresis between moisture uptake and desorption. Human nails showed a lower moisture regain and a much lower diffusion coefficient with respect to human hair. Permeability, directly related to the diffusion coefficient, increased with the degradation treatment. The diffusion coefficient was important in determining the integrity of keratin fibers. (C) 2010 Elsevier B.V. All rights reserved.