Thermal evolution of fluorescence intensity correlation functions of single molecules of terrylene in a p-terphenyl crystal was studied and double-exponential decay at 5 K was found to change to a mono-exponential decay above 17.5 K. Such behavior was attributed to spin-lattice relaxation (SLR), which couples the long-lived zero-field triplet spin sublevel T-Z with the short-lived and experimentally unresolved T-X and T-Y sublevels. For two of the studied molecules, which were probably deformed from planar symmetry, we observed overpopulation of the spin sublevel T-Z and activation of the SLR at temperatures above 20 K. The SLR rate constant exhibited Raman relaxation. (C) 2012 Elsevier B.V. All rights reserved.