Photochemical behaviour of the title complex ( = mercaptosuccinate) was defined as photodissociation and photooxidation– substitution reactions induced by the MLCT transition. The stable products as well as shortly lived intermediate species were identi- fied and characterised by ESR and fast UV/VIS spectroscopic methods. The photodissociation of excited (Scheme1, path a) shifts the equilibrium between the complex and (Eq. (1)). Photooxidation–substitution reaction (Scheme 1, path b) leads to formation of the complex and the radical. The radical generates different NO-donors in secondary thermal processes, which, however, have no noticeable influence on the nitrosation capacity of the system. Moreover, due to the fast reactions in equilibrium between nitroprusside and its thiolate derivative (Eq. (1)), the photooxidation–substitution is really a photocatalytic process and the nitrosation agents are produced mostly at the expense of nitroprusside, whereas the complex behaves as a photocatalyst. Its photoreactivity induced by visible light (=526 nm, =6000) reduces the threshold energy of the process to nearly the phototherapeutic window.