The reactions between NO and CO adsorbed on Rh(100) were studied with temperature programmed reaction spectrometry and static secondary ion mass spectrometry and compared with similar reactions on Rh(111). Elementary steps in the overall reactions, such as dissociation of NO, and reaction between CO and O atoms were studied as well. Dissociation of NO is faster on the more open Rh(100) surface, while formation of Nz is slower. Desorption of either CO or NO occurs at comparable rates on Rh(100) and Rh(111). The oxidation of CO to CO2 proceeds much faster on Rh(100) than on Rh(111). When the Rh(100) surface is saturated with NO and CO, explosive formation of CO2 is observed, which can be explained by an autocatalytic mechanism, in which the availability of empty sites plays a crucial role.