AbstractSelf‐assembled monolayers of cystamine and cysteine chemisorbed on gold electrode surfaces have been used as a support for covalent immobilization of some phenoxazine and phenothiazine derivatives. The covalent attachment was achieved after previous functionalization of the free amino groups on the electrode surface with bifunctional reagents (terephthaloyl chloride and 1,6‐hexamethylene diisocyanate) able to react with the imino form of the amino group in position 3 of the phenoxazine and phenothiazine derivatives.In every case, a shift of the redox potentials between 130 and 180 mV towards more positive potentials, as well as an increase in the peak‐topeak separation between anodic and cathodic peaks was observed after covalent attachment. The surface coverages (10−11<Γ<10−10mol/cm2) obtained from the cyclic voltammograms were those for a not densely packed monolayer and varied depending on the redox compound immobilized. The surface‐modified electrodes obtained were afterwards used for the electrocatalytic oxidation of NADH.UV‐vis spectroscopy was used to monitor the successive steps of surface modification using gold semitransparent electrodes. The total surface coverage of redox mediator on the electrode surface could also be calculated from the optical difference spectra between the oxidized and the reduced state obtained ‘in situ’ in a spectroel