The corrosion inhibitor properties of the 3-amino-1,2,4-triazole (ATR) and 2-aminothiazole (ATH) molecules were studied by means of polarization curves and electrochemical impedance spectroscopy techniques. The experimental results indicate that ATH is a bad or dangerous inhibitor at low concentrations while ATR behaves as an efficient inhibitor at all concentrations. The rationalization of this behavior was achieved by means of first principles theoretical calculations, of the all-electron type, performed with the Gaussian-98 program, at the HF, MP2, B3LYP, and BLYP levels of theory, in concert with 6-31G** orbital basis sets. The reactivity of ATR and ATH was addressed in terms of the computed frontier molecular orbitals, highest occupied molecular orbital and lowest unoccupied molecular orbital, charge distributions, reactivity indices, and electrostatic potentials (EPs). These theoretical parameters indicate that the ATR moiety has much greater inhibitor efficiency than ATH, since, for instance the EP of the former displays two maximums on the nitrogen centers, favorable for the coordination with the metallic centers, while the latter has only one maximum. Analysis of the coordination modes of these two molecules with iron atoms indicates that in ATR-Fe, ATR is bonded to Fe through two nitrogen atoms of the ring yielding a highly stable ATR-Fe system. However, in ATH-Fe, ATH is bonded to Fe only through one N atom, because the S center experiences a repulsive effect with the metal atom. These experimental results suggest that derivatives based on the triazole rings are good candidates for the design of high performance corrosion inhibitors.