The observations by the ion composition instrument (ICI) on the ICE spacecraft made during the encounter with comet P/Giacobini-Zinner (Ogilvie et al., 1986) are discussed in detail. Solar wind 4He++ kinetic temperatures, densities, and velocities before, during, and after the encounter are presented. These data combined with 4He++ velocity distributions provide evidence for the existence of a thick diffuse shock. Relative abundances of water group ions and CO+ are derived along with an estimate of the abundance of an ion with M/Q=24¿1 amu/e. The ICI data are compared with electron data from two other experiments (Bame et al., 1986; Meyer-Verner et al., 1986) and found to be in reasonable agreement in the region outside the tail. Spectroscopic data for several neutral and ionic species are compared with the ICI results for the water group ions and CO+. The spectroscopic data are also used to eliminate Mg+ and CN+ as candidates for the M/Q=24 peak. The two most likely candidates are C2 + and Na +, but neither photoionization of parent neutrals nor sputtering from dust grains is sufficient to explain the observed abundance relative to H2O+.