Phthalazine adsorbed on colloidal silver surfaces is found to convert photochemically to a product in which the N=N bond of the molecule likely breaks to form an adsorbed species resembling an ortho-substituted benzene. The photochemical kinetics was studied using a simple flow cell. The photochemical rate constant was found to be large in the visible region of the spectrum, increasing toward the blue. We show, incidentally, that SERS spectra of phthalazine reported previously by us and by others were heavily contaminated by the spectral features of the photoproduct. Hence previous explanations of the unusual excitation wavelength and coverage dependence are incorrect. The photochemical reaction is found to be a one-photon process; hence, the large absorption cross section in the visible is likely due to a metal to molecule charge transfer transition. (Solution-phase phthalazine is transparent in the visible.) It is likely that a significant number of published SERS spectra of other molecules contain spectral features due to photoproducts. By using dynamic methods such as that described, one can avoid these complications.