The photoreduction by amines and N-phenylglycine, NPG, of six styrylquinoxalin-2(1 H)-ones derivatives substituted in the styryl moiety, R-SQ, was studied by using flash photolysis. The photoreaction is initiated via a single electron transfer from the electron donor (amines or NPG) to R-SQ excited triplet state, 3R-SQ*, with the formation of a triplet state radical ion pair or a charge transfer exciplex, 3[CRIP/CTE]. These species live longer than the respective 3R-SQ* and have very similar transient spectra. In the presence of NPG, these 3[CRIP/CTE] evolve on μs time scale to the respective hydrogenated radicals, R-SQH•, whose transient spectra and reaction rate constants with NPG are reported. The identity of these hydrogenated radicals was supported by the spectra obtained with the α-H donor triethylamine and previous pulse radiolysis studies in 2-propanol. Our findings allow proposing a radical chain reaction mechanism that explains the observed spectral behavior and rationalizes formation of the main product formed by binding of four PhNHCH2• derived from NPG decarboxylation.