Poly(9,9-dioctylfluorene) (PFO) adopts a particular type of conformation in dilute solutions of the poor solvent methylcyclohexane (MCH) below 273 K, which is revealed by the appearance of a red-shifted absorption peak at 437-438 nm. The formation of this ordered conformation depends on the temperature but is independent of polymer concentration over the range studied (3-25 mu g/mL). On the basis of absorption, steady-state, and time-resolved fluorescence data, the new absorption peak at 437-438 nm is assigned to a highly ordered conformation of PFO chains, analogous to the so-called, beta-phase first identified in PFO films. From the study of PFO solutions in MCH as a function of temperature, we conclude that these ordered segments (beta-conformation) coexist with less ordered domains in the same chain. When the ordered domains are present, they act as efficient energy traps and the fluorescence from the disordered regions is quenched. The transition between the disordered and the ordered PFO conformations is adequately described by a mechanism that involves two steps: a first, essentially intramolecular, one from a relatively disordered ( R) to an ordered conformation (,), followed by aggregation of chains containing beta-conformation into anisotropic ordered domains. From the temperature dependence of the 437-438 nm peak intensity, the transition temperature T-beta = 261 K, enthalpy Delta H-beta= -18.0 kcal mol(-1), and entropy Delta S-beta = -68.4 cal K-1 mol(-1) were obtained. The formation of the beta-conformation domains were also followed as a function of time at 260 K. The rate constants at 260 K were determined, showing an order of magnitude around 10(-3) s(-1) (k(alpha ->beta) = 5.9 X 10(-4) s(-1); k(beta ->alpha) = 9 X 10(-4) s(-1); k(agg) 2.3 X 10(-3) M-1 s(-1); k(diss) = 4.4 X 10(-4) s(-1)). This small magnitude explains the long times required for a " complete" conversion to the beta-conformation.