Two-ion component electrostatic ion cyclotron (EIC) waves are studied in a single-ended Q-machine containing variable concentrations of Cs+ and K+ ions. Measurements of the excitation and propagation characteristics of the waves are presented and compared with available theories. The waves are internally excited by drawing an electron current along the magnetic field to an 11-mm exciter disk. Both Cs+ and K+ EIC waves can be simultaneously excited over a large range of relative ion concentrations. The EIC wave features are found to depend on the relative ion concentrations. As the light ion (K+) concentration is increased, the frequency of the heavy ion (Cs+) mode shifts toward its cyclotron frequency. Likewise, the frequency of the light ion mode approaches the light ion cyclotron frequency as the heavy ion concentration is increased. The critical drift velocity for the excitation of the heavy ion (Cs+) mode is more strongly dependent on the relative ion concentrations than that of the light ion (K+) mode. These results are of interest in the context of EIC wave excitation in the ionosphere. ¿ American Geophysical Union 1989