Propagation characteristics of electromagnetic ion-cyclotron waves are extremely sensitive to the relative composition of thermal ions: this can consequently control the instability properties of resonant energetic ions in the Jovian magnetosphere. The dominance of heavy ions in the Io plasma torus will supress ion-cyclotron instability near the equatorial plane. Wave growth, however, can still occur away from the equator in the region where thermal hydrogen is expected to predominate. A theoretical assessment of the convective L-mode gain indicates that Voyager I did not enter the preferred region of instability. Evidence for instability may nevertheless be available since oblique ion-cyclotron waves are expected to experience a natural polarization reversal to the R-mode which is able to propagate from high latitudes to the equator. If the amplitude of the unstable L-mode waves exceed a gamma the concomitant scattering rate of resonant energetic ions should approach the strong diffusion limit. In the absence of significant Landau damping the power spectral density of fluctuating (>10 Hz) R-mode electric fields detectable near the equatorial plane by the Voyager plasma wave instrument could then exceed 10-7 (V/m)2Hz-1 in the inner torus. Such intense ion-cyclotron waves could account for the observed decrease in ion phase space density and the excitation of auroral emissions on field lines mapping from the Io torus.