alpha- and beta-phase formation of poly (9,9-dioctylfluorene) (F8) has been investigated in terms of optical absorption and photoluminescence measurements. beta-phase F8 thin films are prepared by a drop-cast method or by exposing spin-coated F8 thin films to toluene vapor. It is found from film-thickness dependence of PL spectra that alpha phase is formed in spin-coated F8 thin films whose thickness is less than 200 nm, while the coexistence of alpha and beta phases is observed in the thin films with thickness of > 200 nm. (c) 2005 Elsevier B.V. All rights reserved.