Significant gas-phase chemistry occurs in the comae of bright comets, as is demonstrated here for the case of Comet Hale–Bopp. The abundance ratio of the two isomers, hydrogen cyanide and hydrogen isocyanide, is shown to vary with heliocentric distance in a way that is consistent with production of HNC by ion–molecule chemistry initiated by the photoionization of water. Likewise, the first maps of emission from HCO+ show an abundance and an extended distribution that are consistent with the same chemical model.