The electro-oxidation of gold and thiourea (TU) in aqueous acid solutions is investigated by means of in situ FTIRRAS measurements complemented by voltammetry. The formation of soluble species related to the gold(I)-TU complex and TU electrooxidation products is monitored by following the changes in the IR spectra with the applied potential in the range -0.1 -1.5 V. Experimental data show that gold electrodissolution produces soluble [Au(CS(NH₂)₂)₂]₂SO₄ species. The IR spectrum of solid samples of this complex is reported. The electrodissolution reaction begins at approximately 0.15 V (SHE), concomitantly with the early stages of TU electro-oxidation. The electro-oxidation of TU on gold in acid solutions produces formamidine disulphide (FDS) at potentials below 1.2 V (SHE). The formation of the gold complex and that of FDS are quasi-reversible processes, as revealed by FTIRRAS data. Once the formation of the oxygen-containing layer on gold sets in, the electro-oxidation of TU and FDS yields carbon dioxide, sulphate ions and CN-containing products. Adsorbed sulphate species, which are also detected from 1.2 V upwards, exhibit a band shift of 27 cm^-1 V^-1.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas