We investigate the optical properties of Er-Yb-Y disilicate thin films in which Er and Yb contents are varied independently. From time-resolved measurements of Yb decay in absence and in presence of Er, we analyzed and modeled the energy transfer mechanism identifying all the de-excitation channels and measuring a coupling constant CYb-Er = 3.0 × 10−39 cm6/s. Finally, we calculated the efficiency of Yb-Er energy transfer process and we correlated it with Er photoluminescence, demonstrating that it is controlled by the energy transfer mechanism. The maximum efficiency is found for 1.5 × 1021 Er/cm3 and 1.5 × 1022 Yb/cm3, by making this material very promising for optical amplifiers.