Diffusions of Cu2+ and Fe3+ in gelatin generate a moving blue band. It is formed by a diffusion of Cu2+ and a phase separation of gelatin with diffusing Fe3+. The diffusing Fe3+ forms Fe(OH)(3) colloids and gathers gelatin molecules from the surroundings. The diffusion of gelatin molecules generates the concentration gradient, resulting in a gel/sol transition in the dilute phase. In the region where the concentration of Fe3+ is high enough, the gel remains hard, while a sol phase appears under the hard gel. The absorption spectrum of Cu2+ depends on the concentration ratio of Cu2+ to gelatin. As a consequence, we can see a blue band in the restricted region between the diffusing front of Cu2+ and the phase separation front. The movement of the blue band is caused by a coupling of a simple diffusion and the phase separation. (C) 2007 Elsevier Inc. All rights reserved.