This paper describes the role of adatoms in the electrochemical behavior of gold electrodes modified with metal ion tetrasulphonated phthalocyanines (MTSPc). After electrodeposition of MTSPc (M = Cu(II), Fe(III), Co(II), and Al(III)) the long-term stability of the modified electrodes was investigated. It took about 1000 scans before the electrode showed a constant behavior. During the scanning the MTSPc adsorption peak potentials shifted from their initial value to a value of -0.36 V vs. SCE, which is related to the redox reactions of the gold adatoms. After a long scanning procedure, the amount of these adatoms increases and controls the redox reactions of the deposited MTSPc. (C) 2005 The Electrochemical Society.