During the Voyager 1 and 2 encounters with Jupiter in 1979 the positive ion composition of the cold thermal plasma in the Jovian plasma sheet and cold Io torus could be obtained when the plasma Mach number in the spacecraft rest frame was sufficiently large. Clear indication of an ionic species with a mass-to-charge ratio between 16 (O+ and/or S2+) and 23 (Na+) is present in two spectra, one from the Voyager 1 encounter and one from the Voyager 2 encounter. Both of these spectra were acquired during apparent crossings of the plasma sheet in the middle magnetosphere. One interpretation is that the most likely identification of the ion is K2+, even though there is no evidence in the data for K+. Such an ion presence would be consistent with reports of neutral potassium associated with the Jovian system and originating at Io as well as with the identification of an ion with a similar mass-to-charge ratio at higher energies in the Ulysses data. An alternative interpretation is that the ion is a singly ionized molecular species associated with water products sputtered from Europa, a scenario which may also be consistent with the Ulysses data. However, other species present in the data and the low temperatures favor the Iogenic potassium hypothesis. ¿ American Geophysical Union 1993