The kinetics of the photochromic first-order reaction merocyanine --> spiropyran in PPMA (Macromolecules 1988, 21, 923) is reanalyzed. We apply a method which is conceptually analogous to a technique for discriminating between homogeneous or heterogeneous dynamics on the basis of generalized time correlation functions. The requirement for such an analysis is to selectively prepare a fast or slow subensemble of relaxation sites and to monitor their dynamics relative to those of the entire ensemble, which has been realized experimentally for an isomerization reaction probing the potential barriers imposed by the polymer matrix. For the case of heterogeneous dynamics expected here, a prediction can be made for the decay related to a subensemble of the fast relaxors which does not involve assumptions about the extent of the rate memory or, equivalently, of the temporal fluctuations of activation barriers.