In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm(-1). It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 V-RHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 V-RHE and implies the formation of [Au(I)-(TU)(2)](+), which is oxidized to NH2CN and S-0 atE > 0.80 V-RHE. In a following oxidation step at E > 1.20 V, N-2, CO2, and HSO4_/SO42- were produced. The second parallel reaction occurs from TU in solution at E > 0.50 V-RHE to form (TU)(2)(2+). All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.