We report the effects that geometrical confinement has on both the glass transition temperature, Tg, and the crystalline phase transition temperature, Tc, for two widely studied conjugated polymers, namely poly(9,9-dioctylfluorene) [PFO] and poly(9,9-dioctylfluorene-co- benzothiadiazole) [F8BT]. The Tg and Tc values were determined for thin film samples via temperature-dependent ellipsometry measurements. The thickness-dependent Tg (Tc) behavior is characterized by three regimes, namely, (i) large d or bulk samples for which Tg = Tgbulk (Tc = Tc bulk). (ii) intermediate d samples for which Tg > Tgbulk (likewise for Tc), and (iii) ultrathin samples for which Tg drops again (likewise for Tc). The intermediate regimes occur for 160 nm > d > 60 nm and 300 nm > d > 80 nm for PFO and F8BT, respectively. The higher-than-bulk Tg and Tc values offer the potential to design more robust and thermally stable polymer optoelectronic devices, including light-emitting diodes, lasers, and solar cells. © 2006 American Chemical Society.