Surface-lattice specificity of electrosorption of H and OH or O species, of anions and of electrocatalytic oxidation of small molecules is of major interest in electrochemical surface science. The present paper reports new examples of lattice-specific electrosorption, of the small molecular cation, guanidonium, the behaviour of which is highly specific to the 2-d lattice geometries of Pt(111), (110), (100) planes and the (511) stepped surface. The guanidonium adsorbate is of special interest because it is a resonating valence-bond cation. The effects are manifested as striking changes in the cyclic voltammograms for the above surfaces in acid and alkaline solutions caused by the presence of the above ion and in displacement-current transients arising from competitive adsorption by CO admitted to the Pt surfaces at various constant electrode potentials. Although the organic adsorbate is a highly stable cation, it has effects on the H upd and electrolyte-anion current profiles in cyclic voltammetry that are reminiscent of anion adsorption. It is suggested that such effects must arise from ion-pairing between the guanidonium cations and already adsorbed anions, e.g. HSO4-, ClO4-, leading to reversal of the normal effects of the latter type of ions at Pt.