The two tautomeric forms of 1,4-dihydrazinophthalazine sulfate have been studied by means of density functional theory calculations. The computed structural parameters agree very well with the experimental values of the related crystal structure. The SERS spectrum of 1,4-dihydrazinophthalazine sulfate was then recorded at different solution concentrations and discussed on the basis of the SERS "surface selection rules" in conjunction with the results of theoretical calculations. 1,4-Dihydrazinophthalazine sulfate was found to be mainly chemisorbed on the Ag colloid and to not undergo photochemistry to a photoproduct as reflected by the SERS spectra and calculations. At about 10(-6) mol L-1 the molecular skeletal plane is probably oriented perpendicular to, or at least significantly tilted with respect to, the Ag surface plane. With the increase in solution concentration, a surface coverage with 1,4-dihydrazinophthalazine sulfate may occur.