The irreversible adsorption of methylamine at platinum single-crystal electrodes has been studied by a combination of electrochemical and in situ spectroscopic techniques. From purely electrochemical data, it has been stated that the adsorption of CH3NH2 molecules results in significant alteration of the Pt(h,k,l) adsorption capabilities. Voltammetric experiments show that the methylamine adlayer undergoes different surface processes depending on the orientation of the metallic substrate. This result contrasts intensely with the uniformity of the main product obtained from the open-circuit adsorption procedure, which has been identified as adsorbed cyanide for all surfaces. As concluded from the spectroscopic results, the interaction of methylamine with the platinum surfaces leads to the dehydrogenation (probably full dehydrogenation) of the molecule and to the formation of an adsorbed cyanide adlayer, which exhibits different surface reactivity on each basal surface.