The energy transfer mechanisms between Er3+ and Yb3+ ions have been investigated in LiLa9(SiO4)6O2 under selective Er3+ excitation. IR emission spectra, measured in the CW excitation regime, were used to establish a relationship between the macroscopic transfer and back transfer parameters. These measurements were combined with the results obtained under pulsed excitation to quantify the absolute values of transfer (Yb3+ → Er3+) and back transfer coefficients (Er3+ → Yb3+), C25 = 9.5 × 10−17 cm3s−1 and C52 = 1.4 × 10−17 cm3s−1, respectively. Additionally, it has been observed an energy transfer that reduces the quantum efficiency of the green emitting Er3+ levels. The corresponding macroscopic coefficient has been also determined (CGQ = 6.1 × 10−17 cm3s−1).