In situ IR spectroscopy was used to study NO adsorbed during the NO + CO reaction on two Rh/SiO2 catalysts with 31 and 127 Angstrom crystallite sizes. The reaction was studied at T < 550 K and at variable partial pressures of CO and NO below 0.08 atm. Under these conditions, the reaction is selective to the formation of N2O and the only NO adsorbed species observed by IR is an anionic species (NO-). It was found that the N2O selectivity and the rate per gram of catalysts were similar on both catalysts, resulting in a lower turn over number (TON) for the catalysts with smaller crystallite size. An analysis combining kinetics and infrared results indicates that the reaction rate is determined by the different dissociation rate constant of the adsorbed NO species on each catalyst.