Photochemical etching and deposition systems involve a range of laser‐induced and spontaneous processes occurring on rough, chemically heterogeneous surfaces. As such, they present an interesting opportunity for extending our understanding of surface photophysical and photochemical phenomena. In this article results of recent experiments investigating the effect of illumination on adsorption, dissociation, and desorption in such systems are described. Studies carried out under realistic film deposition and etching conditions are compared to studies of submonolayer assemblies on clean crystalline surfaces.