The electrochemical properties of one phenothiazine (Methylene Blue) and one phenoxazine (Nile Blue) dye immobilized on zirconium phosphate were investigated by incorporation into a carbon paste electrode. The dyes presented a very stable redox property. The midpoint potentials (E-m) were 150 and 50 mV vs. SCE for Methylene Blue and Nile Blue, respectively. The composition and pH of the supporting electrolyte did not affect the E-m, in contrast to those observed for the dyes in solution phase or when adsorbed on carbon. These electrodes showed a good activity in a test made for electrocatalytic oxidation of NADH. (C) 1997 Elsevier Science S.A.43112327