An experimental and theoretical investigation of the electrostatic wave propagating across the magnetic field at frequencies around the ion-ion hybrid resonance is presented. Firstly, the dispersion relation for the warm electron mode ( omega /k/sub ///<< nu e) is measured in a simple toroidal plasma, which is composed of helium and neon ions, with the ion concentration ratio controlled. The experimental results and the calculations reveal that even a small fraction ( approximately 1%) of the minority concentration leads to an additional cutoff and hybrid resonance near the minority-cyclotron-resonance frequency. Secondly, the ion-ion hybrid resonance cone of the warm electron mode is observed for the first time. The dependence of the resonance cone angle on the frequency and the ion concentration ratio is observed to agree with the theoretical predictions.