Acetonitrile in dilute aqueous solution is reactively chemisorbed at Pt. The processes and chemisorbed species that are involved are studied by means of potential difference infrared spectroscopy in conjunction with cyclic voltammetry. In previous work, the latter technique led to a suggested mechanism of the sequential stages of the reactive chemisorption. The present spectroscopic work examines this mechanism by means of a vibrational structural analysis of the potential-dependent adsorbates present on well-ordered Pt(111) and (100) surfaces. In accordance with the mechanism proposed from electrochemical studies, the infrared spectra show, at potentials below 0.35 V vs. RHE on Pt(111), the presence of a reduced form of chemisorbed acetonitrile. This is supported by the observation of a C=N stretching frequency at 1630cm(-1) which is downshifted to 1614cm(-1) upon deuteration. A similar, albeit weaker, spectral feature was obtained upon reduction of adsorbed acetonitrile on Pt(100), which also develops with time, indicating slow reduction as suggested on electrochemical grounds. Overall, the results illustrate the still seldom exploited virtues of combined infrared-electrochemical measurements as an aid to the elucidation of adsorbate reaction pathways.