In this work, we study the mechanism of peptide photodissociation by ultraviolet irradiation at 193 nm wavelength and discuss the role of ionization proton in this process. We found that substituting the ionization proton for the alkali metal cations (sodium) in the peptide ions results in significant changes of the photodissociation spectra. The experimental data obtained in this work revealed that the photodissociation process can be described using the mobile proton model introduced earlier for peptide collision dissociation. The results can be used in proteomics research for optimization of mass spectrometer's parameters to increase the efficiency of peptide dissociation and in developing sequence-specific models for peptide fragmentation prediction.