A theory of light propagation through the samples of molecules having anomalously long-lived photoexcited states is developed with an emphasis on the sample size. Application of the theory to experiments on a specific biomolecular system (redox reactions of the secondary acceptor of the bacterial photosynthetic reaction centre) leads to essential quantitative and qualitative re-evaluation of the results of conventional optico-spectroscopic determination of the reaction rates. (C) 2007 Elsevier B.V. All rights reserved.