The present work is the first report of the use of surface-enhanced Raman scattering (SERS) for two-dimensionally spatially resolved, chemically selective monitoring of the course of a surface-catalyzed photoreaction. We have recently developed the surface-enhanced Raman imaging (SERI) technique, which is chemically selective, has monolayer sensitivity, and can be used under ambient conditions. p-Nitrobenzoic acid (PNBA) deposited on an electrochemically roughened silver surface was used as a model system. In order to examine the dependence of the photoreaction on the state of aggregation of the molecules, two types of samples were prepared, one in which the compound was uniformly adsorbed as a monolayer and another in which there was a combination of monolayer plus crystallites. In the form of a uniformly adsorbed monolayer, the SERS image (based on the 1348 cm(-1) band) exhibited a relatively uniform intensity level over the sample surface, due to uniform roughness, but there were slight variations that are related to small differences in the surface roughness, as measured with atomic force microscopy (AFM). A photoreaction, induced by the Arf illumination (514.5 nm), involving PNBA is known to be strongly catalyzed by metallic silver and is strongly suspected to produce azodibenzoate. The extent of this photoreaction, which was successfully mapped using SERI, was found to depend upon whether the PNBA was in the form of a monolayer or multilayers (i.e., crystallites). The time dependence of the SERS intensity clearly shows that the photocatalytic reaction, as monitored by both the 1348 cm(-1) reactant peak and the 1437 cm(-1) product peak, takes place to a much larger extent (similar to 80%) when PNBA is adsorbed as a monolayer, while the reaction proceeds to a much lesser extent (10-20%) when the compound is present as small crystallites on the surface, even for the molecules that are in direct contact with the Ag surface, due to steric limitations. However, work of others (Bercegol, H.; Boerio, F. J. J. Phys. Chem. 1995, 99, 8763-8767) has shown that PNBA molecules in the form of an adsorbed layer but separated from the Ag surface by similar to 1.8 nm can still undergo photoreaction, showing that an electromagnetic-type mechanism may also be operative for surface-catalyzed photoreactions.