Ion composition measurements made in the environment of the comet Giacobini-Zinner by the International Cometary Explorer (ICE) spacecraft have shown that H2O+ is a major ion species, together with CO+ ions1. The presence of these two cometary ions has long been evident in optical spectra of cometary ion tails2. However, the ICE observations were surprising in that they detected ions of mass 23-24 AMU with a relatively high abundance1. According to the experimenters, these ions may be either Na+ or C2 +, if not both. We suggest here that the ions detected may indeed be in part Na+ and/or Mg+ and that these and other metallic ions (especially Si+ and Fe+) may be an important component of the cometary ionosphere and central plasma tail. Our reasons are similar, in principle, to those which account for the prevalence of such ions in sporadic E layers in the terrestrial ionosphere, notably the comparatively short timescales for ionization of their neutral parent atoms and the large difference between the rates of dissociative and radiative recombination 3,4.