The exchange reaction of adsorbed formate on Ni 110 with formic acid in gas phase was studied by the time-resolved ลฝ . ลฝ ลฝ. . infrared reflection absorption spectroscopy TR-IRAS . It was found that the pre-adsorbed deuterated formate DCOO a ลฝ . ลฝ . was changed to HCOO a in the presence of HCOOD g at the temperature region from 240 to 300 K. In such a temperature region, the formate was stable on the surface in vacuum and the decomposition of formate was not observed. It was ลฝ . suggested that the pre-adsorbed DCOO a was desorbed as DCOOD accepting deuterium atom from HCOOD. The activation energy of the exchange reaction was estimated to be 23 " 2 kJ mol y1 . The reaction order with respect to formic ลฝ . acid pressure was 0.3 " 0.1, while that as to the coverage of DCOO a , was unity. Taking into account all the experimental results, the exchange of pre-adsorbed formate with post-exposed formic acid is suggested to proceed via forming a hydrogen-bonded intermediate complex on the formate's site, because all the sites were already occupied by formate. The small activation energy of the exchange reaction is discussed on the basis of 'adsorption-assisted desorption'. We propose that when desorption and adsorption are coupled, the activation energy of the exchange reaction should be described as a sum of the potential energy profiles of a desorbing and an adsorbing molecule.