Rh/SiO2 model catalyst surfaces are prepared under ultra-high vacuum conditions and examined in situ using scanning tunneling microscope and CO infrared reflection absorption techniques, to quantify the number and kinds of active Rh surface sites available for kinetic reaction (CO oxidation) as a function of Rh particle size. The results are compared against CO desorption measurements and elevated pressure CO oxidation reaction kinetics, to evaluate the extent of the correlation between the low and elevated pressure site characterization techniques. Data demonstrate that estimates of Rh active sites exhibit good agreement between the characterization methods and illustrate the utility of low pressure surface science characterization techniques in understanding elevated pressure reaction kinetics on model catalyst surfaces.