The lifetimes of Gd3+6P7/2and Yb3+2F5/2fluorescence in a glass host have been studied at room temperature as a function of concentration, and in the case of Gd3+as a function of added Eu3+concentration. The results show that the ions ``concentration quench'' by a mechanism involving the migration of excitational energy from ion to ion until a sink in the structure is reached. This result is significant to the preparation of laser materials.