Fluorescence blinking of single Dil molecules was studied in matrices: polymethylmetacrylate (PMMA), polystyrene (PS), and polyvinyl alcohol (PVA). The triplet lifetimes of Dil were signicantly different in PMMA, PS, and PVA. We found that short triplet lifetimes give short lifetimes of photobleaching, which is attributed to fluorescence quenching by molecular oxygen. We also found, that the intersystem crossing yield changes, for the same chromophore, when the polymer host is changed; a feature which could be attributed to different internal conversions for different polymer matrices. Long-lived dark states were often observed in PVA, a feature which suggests that dark state formation is due to a radical formation by intermolecular electron transfer. (C) 2008 Elsevier B. V. All rights reserved.