A chemically modified electrode constructed by mixing nickel tetrasulfonated phthalocyanine (NiTsPc) immobilized on modified silica gel with carbon paste showed a redox process with a midpoint potential of 0.44 V vs see. Experiments carried out with different supporting electrolytes suggested an effect due to the nature of the anion. The solution pH does not affect the midpoint potential but the peak current increases when the pH is decreased. The immobilization procedure causes an increase in the monomeric form of the complex. The electrochemical property of NiTsPc adsorbed onto modified silica showed good stability even in acidic media (pH = 2) and the ability to catalyze the electro-oxidation of cysteine at 0.5 V vs see. (C) 1998 Elsevier Science Ltd. All rights reserved.434160916651673