Trace concentrations of metallic impurities present at the ppm level in conjugated polymers enable highly efficient radiative triplet decay under both optical and electrical excitation. We demonstrate that the phosphorescence yield is governed by both diffusion of triplets to metallic sites as well as non-radiative triplet decay. Both the triplet mobility and non-radiative decay are enhanced at elevated temperatures. As triplet emission occurs from a small number of emitting sites, the relative dominance of these two processes depends sensitively on triplet density. Two extreme regimes of triplet density are probed by optical and electrical excitation. Unusual thermochromic properties of triplet excitons are also discussed. (C) 2003 Elsevier B.V. All rights reserved.