Abstract Methanation on Ir surfaces at low gas pressure (∼10 −8 Torr) and low temperature (150 K) has been observed using pulsed-laser imaging atom probe. The reaction intermediates can be detected in the pulsed-laser field desorption time-of-flight mass spectrum, thus it is possible to study the reaction steps in detail. Based on our results, it is reasonable for us to conclude that formation of methane at low gas pressure and temperature is through hydrogenation of surface carbon dissociated from carbon monoxide on iridium surfaces. We have also studied the pressure dependence of production of CH 4 and CO 2, and observed oscillations in the signal intensities of CH 4 + and CO 2 + with a time period of ∼8 s under the condition of our experiment. It is noted that the oscillation of the signal intensities of CH 4 and CO 2 has a 180° phase difference.