We report the first Thomson-scattering measurements of the growth of ion-acoustic waves in well-characterized multi-ion-species plasmas consisting of gold and beryllium. We observe that only the berylliumlike mode grows, verifying linear kinetic theory. In addition, a twofold increase in ion temperature is measured when ion-acoustic waves are excited to large amplitudes by stimulated Brillouin scattering (SBS). This increase in ion temperature is a strong indication of hot ions due to trapping. We explain the measured SBS reflectivity by nonlinear detuning of the SBS instability due to these trapping effects.