Photochemical interaction of cyanocobalamin and hydroxocobalamin with cysteine

•Photochemical interaction of vitamin B12/B12b in presence of cysteine (CYS) has been confirmed by spectroscopic methods.•Kinetics of photolysis of B12/B12b ̶ CYS has been studied.•B12 and B12b ̶ CYS have been determined by spectrometric method.•The Mode of photochemical interaction of CYS and B12 and B12a/B12b has been discussed.•Quantum yields of photolysis of B12 and B12a/b have been determined.