This article describes the glutathione-triggered disassembly of gold nanocomposites composed of gold cores and water-soluble copolymers [poly(N-n-isopropylacrylamide-co-acryloyldiethyletriamine)] attached to the surfaces of gold cores. The gold nanocomposites exhibit a bluish purple color because of the assembled gold cores that are conjugated with the diethylenetriamine groups incorporated into the copolymers. Glutathione added to the gold nanocomposite solution adsorbs onto the surface of the gold cores to liberate diethylenetriamine groups, resulting in spontaneous disassembly that changes the color of the solution to a reddish shade. Increasing the glutathione concentration facilitates the spontaneous disassembly of the gold nanocomposites. For the determination of glutathione, the colorimetric change of the gold nanoparticles is quantified with the a* value of the L*a*b* color coordinates defined by the CIE (Commission In de l'Eclairage) chromaticity diagram. A linear relationship between the a* value and the glutathione concentration of up to 6 x 10(-6) is obtained 15 min after the addition of glutathione that has a detection limit (defined as 3 sigma) of 2.9 x 10(-8) mol/L. The colorimetric assay is successfully applied to the determination of glutathione eye drops and health supplements.