Keywords: Oxidoreductases / Redox chemistry / Electrochemistry / Ab initio calculations / StructureϪactivity relationships N-Arylhydroxamic acids (AHAs) are promising novel N−OH mediators for oxidoreductase catalysis. They are electrochemically active compounds with a redox potential of 0.31−0.41 V vs. SCE. Representative oxidoreductases, e.g. fungal peroxidase from Coprinus cinereus (rCiP), catalyze the oxidation of AHAs with apparent bimolecular constants (k ox ) of 7.1·10 3 to 1.5·10 7 m −1 s −1 at pH = 8.5 and 25°C. The limiting step in substrate oxidation was the reduction of compound II (Cpd II). The oxidation constants of N-hydroxyacetanilide (1a) and N-hydroxy-N-phenylbenzamide (2a), determined by a stopped-flow and steady-state method, were similar. The decrease in the reduction rate of Cpd II reduction rate decrease occurred at pK a = 8.5 for 1a and 7.7 for 2a. The nitroxyl radical of 1a, an intermediate in the oxidation, shows decreasing stability at alkaline pH. The structure−activity relationships (SARs) of these AHAs were analyzed within the