This study examines the underlying nature of the green emission band observed as a result of oxidation in dialkyl-fluorene polymers. Specifically, we set out to further determine whether an inter- (excimeric) or intra-molecular fluorenone-based excited state is involved. The emission properties of poly(9,9-dihexylfluorene) dispersed at low concentration in a solid polystyrene matrix are carefully explored. In situ, time-resolved photoluminescence measurements are made during photo-oxidation of the blend and during subsequent exposure to an atmosphere saturated with the vapour of a good solvent. The polystyrene matrix suppresses the appearance of the green emission band during oxidation but the subsequent solvent vapour exposure then activates it. The same effect (activation of the green emission) can be achieved by thermally annealing the matrix above its glass transition temperature. Moreover, the activation of the green emission can be reversed by dissolving the film and re-casting. This behaviour is attributed to controlling the phase structure of the polyfluorene/polystyrene blend and is considered strong evidence for an origin of the green band emission in the formation of excimer-like states between co-facially arranged fluorenone moieties. The photoluminescence behaviour of 9-fluorenone and fluorene molecular mixtures in solution is also studied. This model system allows analysis of the green emission band independent of relative intra- and inter-molecular energy transfer effects since this system is affected only by inter-molecular energy transfer. These results provide further evidence for an excimeric origin of the green emission.