From a synthesis of data from the Plasma Science and Ultraviolet Science instruments on the Voyager 1 spacecraft we present a radial profile of O++ abundance between 4.9 and 42 RJ. We observe a sharp rise in O++ mixing ratio near 7.5 RJ, coincident with a sharp rise in effective electron temperature at the outer boundary of the Io plasma torus. Beyond 8.5 RJ the O++ mixing ratio is found to be roughly constant which indicates freezing of the ionization prevailing at the outer edge of the hot torus. ¿American Geophysical Union 1992