The triplet state behaviour of the alternating copolymer poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)-alt-2,5-terthiophene] (PF3T) was studied in benzene solution. This shows a transient absorption at 730 nm, which decays with a lifetime of 10 mus. Using the singlet depletion method, a quantum yield phi(T) = 0.33(5) was determined. This efficient intersystem crossing results from extensive spin-orbit coupling by the sulfur atoms. In the presence of air, the triplet state sensitises singlet oxygen formation in nearly quantitative yield. A lowest triplet state energy of 1.84 eV was determined from time-resolved photoacoustic calorimetry. This is intermediate between polyfluorenes and terthiophene, in agreement with theoretical predictions. (C) 2004 Elsevier B.V. All rights reserved.