THORELLO is a steady-state toroidal plasma, produced in different neutral gases by hot filament electron emission and acceleration by a constant bias voltage. Electron temperature and density, evaluated by Langmuir probes, are of the order of 5 eV and 1010 cm-3, respectively. The maximum toroidal magnetic field on axis is about 2 kG. In this environment, the dispersion relation of electrostatic ion cyclotron waves has been studied, in hydrogen and helium plasmas. These waves are excited by a pair of thin metallic blades, fed with opposite phase by a low power (<10 W) r.f. signal in the range of the ion cyclotron frequency and its first harmonics. The antenna system, installed at the plasma center, in conjunction with a movable r.f. probe, forms a continuous-wave interferometer suitable for the experimental reconstruction of the ion wave dispersion relation. A phase comparison method, for any give frequency, allows the evaluation of the perpendicular wavenumber. The fit of experimental data to the theoretical curves, generated by a numerical analysis of the complete e.s. dispersion function, gives the possibility of evaluating the order of magnitude of the ion temperature. A good agreement is obtained in the hydrogen case by applying the stationary phase method to the normal mode k/sub /// integral, while, in helium, by considering collisional effects, not negligible in weakly ionized, low temperature plasmas.