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• A complex voltammetric study of Sudan I and products of its redox transformations. • The first comparative study of voltammetric behavior of major Sudan I metabolites. • Distinction of Sudan I and its major metabolites by voltammetry is possible. • Adsorption of analytes on basal plane PGE enhances sensitivity compared to BDDE. • Sudan I and its metabolites can be detected in excess of NADPH. Sudan I (1-(Phenyldiazenyl)naphthalen-2-ol) is an orange industrial azo dye. It is utilized as a marker substrate in studies of metabolic transformation of xenobiotics through processes involving cytochrome P450 hydroxylase. In this complex study, redox and subsequent chemical/electrochemical reactions of Sudan I and its three major metabolites (4′‑hydroxy, 6‑hydroxy, and 4′,6-dihydroxy derivatives of Sudan I) were investigated using boron doped diamond and basal plane pyrolytic graphite electrodes. Cyclic voltammetry (CV) in Britton – Robinson buffer pH 7 showed distinct differences in the number and potential of signals, depending on the number and positions of the hydroxy groups. While in CV scans initiated in anodic direction, primary oxidations of paired or unpaired hydroxy groups are dominating, cathodic scans lead primarily to reduction of the azo group and to cleavage of the parent compounds and formation of 1-aminonaphthalen-2-ol and aniline or their corresponding hydroxylated derivatives. These are oxidizable in reverse anodic scans. Combinations of CV scans can be used to distinguish among all tested compounds. Basal plane pyrolytic graphite electrode provides better sensitivity than boron doped diamond electrode due to enhanced adsorption of the aromatic analytes/intermediates. The possibility of detecting Sudan I and its metabolites in the presence of excess amounts of NADPH as an essential component of enzymatic hydroxylation systems, was tested. Although the oxidation signal of NADPH partially interferes with signals of primary oxidation of hydroxy groups (particularly for Sudan I), our results suggest that electrochemical assays could be applicable in monitoring transformation of Sudan I in biochemical studies. [Display omitted] [ABSTRACT FROM AUTHOR]