The d-f emission from Ce3+ and Pr3+ in garnets is attracting considerable attention, especially in relation to application in white light LEDs and scintillators. An important aspect is the luminescence quenching temperature TQ. It is not trivial to determine TQ and to unravel the quenching mechanism. In this paper the TQ of d-f emission for Ce3+ and Pr3+ are determined by temperature dependent lifetime measurements. The results show a TQ for Pr3+ of 340 K for Y3Al5O12:Pr3+ (YAG:Pr) and 680 K for Lu3Al5O12:Pr3+ (LuAG:Pr). For Ce3+ the TQ is too high to measure. An onset of quenching above 600 K (YAG:Ce) or 700 K (LuAG:Ce) is observed. The differences in TQ between YAG and LuAG are explained by a smaller Stokes shift for the d-f emission in LuAG (∼2300 cm−1) compared to YAG (∼2750 cm−1) derived from low temperature luminescence spectra. The large difference in TQ between Ce3+ and Pr3+ is related to the smaller energy difference between the lowest energetic fd state of Pr3+ and the next lower 4f2 state (3P2) compared to the 5d – 4f1(2F7/2) energy difference for Ce3+. Both observations are consistent with luminescence temperature quenching by non-radiative relaxation from the 5d state to the 4f state described by a configurational coordinate diagram and not by thermally induced photoionization.