Evidence for photo-induced radical disulfide bond scrambling in the gas phase during matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is described. The phenomenon was observed during the analysis of tryptic peptides from insulin and was confirmed in the determination of disulfide bonds in the rhamnose-binding lectin SEL24K from the Chinook salmon Oncorhynchus tshawytscha. A possible mechanism for this surprising scrambling is proposed. Despite this finding, the disulfide bond pattern in SEL24K was assigned unambiguously by a multi-enzyme digestion strategy in combination with MALDI mass spectrometry. The pattern was found to be symmetrical in the tandem repeat sequence of SEL24K. To the best of our knowledge, this is the first report of disulfide bond scrambling in the gas phase during MALDI-MS analysis. This observation has important ramifications for unambiguous assignment of disulfide bonds.