The detection and analysis of molecular layers on surfaces can be based on a measurement of the dispersion relation of surface plasmons. In order to reduce the amount of adsorbed molecules needed they should be locally concentrated. This requires the knowledge of the behavior of plasmons for lateral variations in the dielectric part of the interface. A SNOM set‐up is presented to study plasmon propagation. A model study for a structured PMMA layer on a silver surface gives insight into the typical plasmon behavior. © 2003 American Institute of Physics