Hydrogenation of ethylene on a Pt(111) surface in the presence of gaseous ethylene and hydrogen in the 10(3) Pa range was investigated by infrared reflection absorption spectroscopy (IRAS). The IRA peaks assigned to di-sigma-bonded ethylene and ethylidyne were observed on Pt(111) under the flow of 1.3 x 10(2) Pa of ethylene and 6.5 x 10(3) Pa of hydrogen at 200-300 K, and the apparent activation energy of ethylene hydrogenation was estimated to be 37 +/- 5 kJ mol(-1) from the rate of temperature-dependent ethane production. The ethylene hydrogenation on ethylidyne-covered Pt(111) was also examined under the same condition. The peak due to the di-sigma-bonded ethylene was not observed on ethylidyne-covered Pt(111), although the similar rate of hydrogenation and the similar activation energy (39 +/- 5 kJ mol(-1)) were obtained as those on clean Pt(111). The pi-bonded ethylene, which has been previously proposed as an intermediate by the sum-frequency generation (SFG) experiment under the catalytic hydrogenation condition and by IRAS under the low-temperature condition, was not detected in the present study. The similarity of the rate and activation energy for clean and ethylidyne-covered Pt(111) surfaces suggests that-the rate of hydrogenation does not depend on either coverage of di-sigma-bonded ethylene or ethylidyne. The role of the observed species in the hydrogenation of ethylene is discussed.