In our previously published paper we showed how guanidonium (G(+)) ion causes substantial, surface-specific displacements of the H underpotential deposition (UPD) voltammetric current profiles, at Pt(1 1 1), (1 0 0), (1 1 0) and (5 1 1) stepped surfaces, to less positive potentials. This behaviour was attributed to ion-pairing between adsorbed ions of the electrolyte (HSO(4)(-), ClO(4)(-) or OH(-)) and the resonant G(+) cation. Comparatively, the adsorption behaviour of N,N-dimethylguanidonium (DMG(+)) cation at Pt(1 1 1) surface, in contact with 0.5 M H(2)SO(4), was also reported. The present work reports cyclic voltammetric results of comparative experiments, at the same (hkl) surfaces of Pt, in the presence of acetamidine (AA) molecule having structure related to that of guanidine (G). Acetamidine cations cause similar displacements of the voltammetric profiles for underpotential deposition and desorption of H as does G(+), in ways characteristic of the Pt surface geometry. However, in some cases recorded voltammetric profiles are unique to acetamidine and as such they reflect differences between the resonant cations structures of G(+) and AA(+). In addition, current results strongly support the previously proposed mechanism of cation/anion interaction in the double-layer interphase. (C) 2008 Elsevier B.V. All rights reserved.