Time-resolved rapid-scan FT-IR spectra of ethylene hydrogenation over alumina-supported Pt catalyst at 473 K were recorded under continuous H 2 /N 2 flow (1 atm) and pulsed release of C 2 H 4 (30 millisecond duration). Two surface species were observed, namely ethylidyne (CH 3 CPt 3 ) with peaks at 2880 and 1339 cm -1 (lifetime 300 ( 50 ms), and a substantially shorter-lived intermediate with an intense band at 1200 cm -1 and weak absorptions in the 2875-2860 cm -1 region (lifetime around 100 ms). Comparison of the C 2 H 4 + H 2 results with those of experiments using D 2 or C 2 D 4 suggests that the 1200 cm -1 species is a surface ethyl intermediate (CH 3 CH 2 Pt). This is the first observation on the lifetime of surface ethyl species under reaction conditions. The rise of the final ethane product, monitored by the ν(CH) absorption at 2893 cm -1 was found to reach a maximum already in the first recorded time slice. This suggests that the observed CH 3 CH 2 Pt species is a surface-trapped form of the kinetically relevant, only weakly interacting C 2 H 5 radical intermediate.