Formation of methyl formate in comets by irradiation of methanol-bearing ices

► Methyl formate can be formed in irradiated icy samples containing CO and CH3OH. ► Cosmic ion irradiation can account for about 12% of the observed methyl formate in Hale-Bopp. ► Radioactive decay can account for about 6% of the observed methyl formate in Hale-Bopp.