Electrochemical measurements were conducted for investigation of the dynamical process of adsorbed CO formation from CO2 and adsorbed hydrogen on Pt single crystal surfaces in 0.5 M H2SO4. The adopted monocrystal surfaces were Pt(111) and Pt(110) [= 2(111)-(111)], whose atomic orientations are composed of (111) type arrangement on the surface. The rate of the CO formation on Pt(110) was more than 10 times as high as those on Pt(111). Oxidation charge of CO at the final stage of the reaction was almost constant between 0.05 and 0.20 V (vs. rhe) on Pt(110), whereas the amount of adsorbed hydrogen, source material of the reaction, greatly varied in this potential range. This fact suggests that hydrogen atoms are regenerated on Pt(110). Such regeneration was not observed on Pt(111). The difference of the reactivity between Pt(111) and Pt(110) may be attributed to the four-fold hollow site on Pt(110).