The radiative lifetime of single 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine molecules, embedded in a polymer thin film, has been characterized. At room temperature the chemically identical molecules exhibit strong fluctuations in their fluorescence lifetime. The possible conformational origin of these fluctuations has been addressed by semi-empirical quantum-chemical calculations. Specifically, the impact of conformational effects induced by thermal motion on the transition energy and the value of the transition dipole moment has been studied. The results indicate that the strong fluctuations cannot be explained by pure thermally activated conformational changes. Since quenching mechanisms are not responsible for the observed fluctuations, we suggest that molecular segmental dynamics of the surrounding polymer play a key role in determining the lifetime fluctuations. (C) 2003 Elsevier Science B.V. All rights reserved.