The Cassini spacecraft passed within 168 km of the surface of Enceladus on 14 July 2005 during the E2 flyby and passed closer (50 km) during the E3 encounter on 13 March 2008. During both flybys the ion and neutral mass spectrometer (INMS) detected a plume atmosphere mainly composed of water. During the E3 flyby, the INMS measured an ion mass spectrum with a large peak at mass number of 19 Daltons (interpreted as H3O+ ions) but not at the mass numbers of other water group ion species (including H 2O+, OH+, O+). In addition, the INMS observed ion species at mass numbers 36 and 37-possibly the water cluster ion species H2O+-H2O and H3O +-H2O. The INMS detection of cold H3O + in the plume combined with the almost complete absence of cold H2O+ ions is attributed to an active ion-neutral chemistry operating in a plume "ionosphere."