New substrates of horseradish peroxidase and laccase incorporating advantages of ferrocenes, on one hand, and typical aromatic substrates of peroxidase, on the other, viz. p-ferrocenylaniline (FcC6H4NH2-p) and p-ferrocenylphenol (FcC6H4OH-p), have been introduced. The new substrates are superior in terms of (i) high reactivity, i.e., the second-order rate constants for their oxidation by the peroxidase compound II equal ca. 1 × 107 M-1 s-1 at 25 °C, pH 5.0, which are 102 times higher than the corresponding rate constants for oxidation of ferrocene, alkylferrocenes, and aniline; (ii) easy spectral control of redox transformations at 439 or 860−1000 nm (products are colored and the extinction coefficients are high); (iii) the products are water-soluble and do not inactivate enzymes. p-Ferrocenylaniline is readily electropolymerized on a rod carbon electrode in 0.1 M HCl by potential cycling in the range from −0.4 to +1.1 V (versus Ag/AgCl). The peak potentials are weakly pH-dependent in the range 0.5−7 and the slopes of potential versus pH plots are less than 30 mV. p-Ferrocenylaniline is also electrochemically active when adsorbed on a carbon electrode from an acetone solution. It has been demonstrated that the electrodeposited and adsorbed p-ferrocenylaniline films mediate the electro-oxidation of NADH with the sensitivity of 1.4 and 0.25 μA/mM cm2 at pH 5 and 7.2, respectively.