Radical-ion-pair reactions were recently shown to represent a rich biophysical laboratory for the application of quantum measurement theory methods and concepts. We here propose a concrete experimental test that can clearly discriminate among the fundamental master equations currently attempting to describe the quantum dynamics of these reactions. The proposed measurement based on photon statistics of fluorescing radical pairs is shown to be molecular-model-independent and capable of elucidating the singlet-triplet decoherence inherent in the radical-ion-pair recombination process. (C) 2012 Elsevier B.V. All rights reserved.