Alkylidyne-modified (propylidyne or ethylidyne) Pt nanoparticles were electrochemically prepared and characterised by Surface Enhanced Raman Spectroscopy (SERS) and cyclic voltammetry. Formic acid oxidation on alkylidyne-modified Pt nanoparticles was used as probe reaction to evaluate their electrocatalytic properties. SERS spectra show that, in the presence of propylidyne, the observed frequency of nu[CO] is red-shifted with respect to the typical values on polycrystalline Pt in acidic medium, which suggests the existence of an instabilization of the carbon monoxide poisonous species, what contributes to the enhanced electrocatalytic activity. Similar electrooxidation current densities were obtained for both propylidyne and ethylidyne surface modifiers suggesting that the instabilization of the COads seems to be non-dependant of the nature of the alkylidyne intermediates, and that the enhanced mechanism is similar in both cases. The present paper shows how in situ spectroscopic measurements can help to understand the effect of the surface modifiers on the electrocatalytic properties of Pt nanoparticles, which can be effectively applied in real applications. Thus, this experimental methodology will allow selecting those modified-electrocatalysts with better properties. (C) 2009 Elsevier Ltd. All rights reserved.