Thomson-scattering measurements of the growth of ion-acoustic waves are obtained from well-characterized multi-ion-species plasmas consisting of gold and beryllium. Simultaneously, the ion temperature is accurately measured by comparing the relative amplitude of thermal ion-acoustic modes. A twofold increase in ion temperature was 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. The measured SBS reflectivity is explained by nonlinear detuning of the SBS instability due to these trapping effects.