We investigated the temperature dependence of the amplified spontaneous emission ͑ASE͒ of the ␤ phase of poly͑9,9-dioctylfluorene͒. We show that the ASE threshold increases with temperature of about 1 order of magnitude from 17 J cm −2 at 5 K to 206 J cm −2 at 300 K due to the thermal activation of uphill energy migration to nonradiative centers that reduces the population inversion for a given excitation density. The temperature increase also changes the luminescence line-shape switching on ͑around 100 K͒ the ASE from the C-C interunit stretch vibronic replica at the expense of the phenyl-ring quadrant stretch one.

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