The state and the elastic dilatational properties of a mixed spiropyran (with attached hydrocarbon)-octadecanol(1:5, mol/mol) spread monolayer was studied. A localized photochemical excitation was used to generate dilatational motion along the elastic monolayer. The surface pressure Variations at various distances from the photoexcited region were measured. The experimental results are in quantitative agreement with a theoretical model taking into account the rheological properties of the interface and the liquid substrate as well as the rate constant for the photochemical process.