The results discussed in this paper show that modification of the working electrode surface with clay montmorillonite (bentonite) enhances the iodine redox process and electrosynthesis of thyroxine. This is observed from the well defined quasi-reversible redox peaks obtained in the case of a modified surface as compared to bare carbon. These improvements in redox properties are attributed to the effect of pre-concentration and electrocatalysis. Orientation of clay plates also plays an important role.