A generally known energy transfer route in Tb3+ activated phosphors is the D-5(3) --> D-5(4) cross relaxation and the subsequent D-5(4) --> defect impurity quenching. However, this mechanism is not satisfactory to explain the overall feature of measured decay curves of the D-5(3) --> F-7(j) transition in the wide range of Tb3+ concentrations. We suggested a new energy transfer scheme, where the D-5(3) (or) (4) emission is quenched by the second type of cross relaxations from D-5(3) (or) (4) to upper levels such as 5d, charge transfer, and host levels. While neither the rate equation model without the second type of cross relaxations nor Inokuti and Hirayama's equations are well-matched with the measured decay curves, the rate equation model including the second type of cross relaxations is in good agreement with the measured decay curves. (C) 2002 The Electrochemical Society.