We describe a unique method for continuously and independently varying the rate‐controlling time constant of excited Yb+3ions in rare‐earth up‐converting phosphors. We show, in the system BaYb2F8: Er, that at all values of the time constant, the power radiated is directly proportional to the square of the observed radiative lifetime, in accordance with the 2‐transfer model of Auzel. The decrease in this time constant arises from the introduction of radiation‐produced defects which selectively affect the Yb+3rather than the activator ions. The phenomenon can be used to study energy transfer from Yb+3to any activator.