The purpose of this work is to study bismuth deposition/dissolution behavior in an aqueous electrolytic gel media, aiming its application in electrochromism. The eel was made from a polymeric animal protein derivative. This polymer is extremely consistent and it becomes a transparent gel in the 350-850 nm range (visible region). The cross-linking reaction of the gel was made with formaldehyde, targeting to improve the thermal stability of the polymeric matrix without considerable loss of optical properties during electrodeposition. Differential scanning calorimetry and thermo-mechanical analysis were used to verify the transition temperatures of the gel, showing that the cross-linked material can be heated up to 90degreesC without Great changes in their mechanical properties. Voltammetric and chromogenic experiments carried out with the electrochromic device using the eel as electrolyte, were very, promising and the electrochemical reversibility of Bi/Bi3+ redox couple is not affected by the cross-linking reaction showing a large variation of absorbance values. (C) 2004 Elsevier B.V. All rights reserved.