Proton irradiation induced radiolysis of methyl orange was investigated in aqueous solution with an in-air ion beam setup. 566–3024 keV energy protons were used for the irradiations of 1.6 mM aqueous methyl orange solutions, in the presence and absence of tert-butanol radical scavenger. The decoloration of methyl orange was followed by UV/vis spectrophotometry. It was found that the methyl orange can be efficiently decolorized in the applied energy range, caused by degradation processes. Furthermore, the efficiency of methyl orange radiolysis was found to be dependent on the applied proton energy. The irradiations were also carried out in the presence of 0.16 and 0.73 mol dm⁻³ tert-butanol using 3024 keV protons. It was found, that at lower concentration of radical scavenger the degradation was less efficient, while at higher concentration of the radical scavenger, interestingly, the degradation was more efficient when compared to the scavenger-less irradiation experiments. Initial G-values were determined for all ion energies and for different scavenger concentrations.