The coadsorption of CO and NO on Rh(111) at room temperature was studied with scanning tunneling microscopy (STM) in the catalytically relevant range of similar to1 Torr. For gas mixtures where NO is not in large excess, a mixed layer with (2x2) structure is formed. The difference in binding energy between CO and NO on top sites was determined from the measured surface (by direct counting in STM images) and gas mole fractions of each species. A model for the molecular structure is proposed based on the analysis of exchange events between CO and NO molecules in the images. In this model as the partial pressure of NO increases, NO molecules occupy hollow sites first, by displacing CO, and top sites later, where they coexist with CO. As the surface fraction of NO increases, favorable NO-NO interactions cause the formation of segregated NO-rich regions. As with pure NO, a phase transition from the (2x2)-NO to the (3x3)-NO structure takes place in the NO-rich regions at high NO concentration. These results demonstrate the unique ability of STM to obtain molecular-level information under catalytic pressure conditions.