We report on the use of optical probes to determine the thermal transition temperatures of spin-coated films of the prototypical fluorene based conjugated polymer poly(9,9-dioctylfluorene) (PFO). In particular we focus here on the use of temperature dependent photoluminescence (PL) measurements that are well suited to the study of PFO and other intrinsically fluorescent polymers and blends. The integrated PL intensity reveals clear signatures of the glass transition temperature (Tg), the onset of crystallization (T c) and subsequent melting into the nematic liquid crystalline mesophase (Tm). The PL intensity determined transitions are shown to be consistent with an independent optical probe, namely spectroscopic ellipsometry, and with differential scanning calorimetry measurements on bulk samples. The especially strong contrast in PL intensity at Tc and Tm is shown to be a consequence of changes in light out-coupling in the direction of PL detection, a conclusion that is confirmed by measurements of edge emitted waveguided PL and amplified spontaneous emission, and by analysis of ellipsometry data. © 2005 IOP Publishing Ltd.