To investigate the penetration of thioglycolic acid (TG), thiolactic acid (TL), and L-cysteine (CYS), into keratin fibers, cross-sectional samples of virgin white human hair treated with TG, TL, and CYS were prepared. A new method for analyzing the diffusion behavior of reducing agents into human hair was developed. The diffusion pattern of reducing agents into human hair, which cannot be determined by optical microscopy, can be determined by the method developed. The method involves treating virgin hair fibers with TG, TL, and CYS. After the treatment, the cross-sectioned hair samples were dyed with methylene blue and the cross-sectional intensity scans were measured at a wavelength of 664 nm (lambda(max) of methylene blue) with a microspectrophotometer. The three different diffusion patterns from the three reducing agents were obtained. The penetration of TG and TL into virgin human hair clearly increased by increasing the treatment time and pH. On the other hand, the penetration of CYS was less than TG and TL (CYS could not penetrate into the cortex region of the virgin human hair). Also, the diffusion pattern of TG showed Fickian type characteristics. The apparent diffusion coefficient of TG into human hair at pH 9.0 determined from the TG concentration profile was found to be 10(-9) cm(2)/(s). On the other hand, the apparent diffusion coefficient of TG into human hair at pH 7.0 was 10(-10) cm(2)/(s), and thus, the apparent diffusion coefficient of TG depended on the pH of the TG solution. From these experiments, we have concluded that the diffusion patterns of the three reducing agents in this study depended on the electrostatic interaction between the human hair and the reducing agents. (C) 2004 Wiley Periodicals, Inc.