In this study, an attempt is made to characterize the surface of the catalyst by infrared analysis during the hydrogenation of but-1-yne using an IR cell as the reactor. This cell operates in dynamic conditions under a hydrogen hydrocarbon flow and is online with a chromatograph apparatus which allows the extent of the reaction to be monitored. The results are analysed regarding the variation of the infrared bands as a function of the reaction selectivity. Two features are observed in the case of a selective catalyst : (i) the presence of H-bonded butyne species on the support and (ii) the presence, on platinum, of a species characterized by a broad absorption band at 1687 cm-1, which could be assigned, by comparison with organometallic complexes, to an eta2-butyne species. The presence of this latter species leads to a selective catalyst by site blocking. On a nonselective catalyst the surface coverage and the nature of the adsorbed species are totally different, though the same conversions are obtained on both types of catalyst. An interpretation is made considering equilibria between adsorbed species and the virtual pressure of butyne.