The oxidation of aniline with ammonium peroxydisulfate (APS) in aqueous solution of the plant-derived polyphenol, tannic acid, is reported. The effect of initial mole ratios [tannic acid]/[aniline] and [APS]/[aniline] on molecular structure, morphology, conductivity, and electroactivity of synthesized materials (PANI–TA) was investigated by UV–Vis, FTIR, Raman and EPR spectroscopies, scanning and transmission electron microscopies (SEM and TEM), conductivity measurements, and cyclic voltammetry, respectively. PANI–TA nanorods with the average diameter of 70–100 nm and conductivity of 1 × 10−3 S cm−1 were synthesized at [tannic acid]/[aniline] = 0.01 and [APS]/[aniline] = 1, while nonconductive, solid microspheres were obtained at [tannic acid]/[aniline] = 0.1 and [APS]/[aniline] = 2. The mechanism of the early stages of simultaneous oxidation of aniline and tannic acid has been computationally studied by the RM1/COSMO method.