This paper reports the details of a high-pressure and -temperature in situ transmission infrared reactor cell and experimental approaches for investigation of the nature of adsorbates in CO hydrogenation and NO-CO reaction on Rh/SiO2 catalyst. The infrared cell used in this study allows easy assembling and reliable operation up to 773 K and 6.0 MPa. The structure and coverage of adsorbates during reaction are determined by an infrared spectrometer, and the composition of gaseous effluent from the infrared cell is monitored by a mass spectrometer. The steady-state (CO)-C-13 step transient shows that gaseous CO rapidly exchanges with adsorbed CO, which is slowly converted to CH4 during CO hydrogenation at 513 K and 0.1 MPa. The pulsing CO study reveals that linear CO is more reactive than bridged CO during methane and CO2 formation, and bridged CO sites are blocked from CO disproportionation. Steady-state (CO)-C-13 pulse transients show that the CO2 response leads the CO response, and Rh-NCO and SI-NCO are not involved in the formation of CO2 from CO during NO-CO reaction. The advantages and limitations of the in situ infrared and transient approaches for catalysis research will be discussed.