Polyhedral oligomeric silsesquioxanes (POSS) anchored to poly(2-methoxy-5-(2-ethylhexyloxy)-1.4-phenylenevinylene) (MEH-PPV) (MEH-PPV-POSS), and to poly(9,9-dihexylfluorenyl-2,7-diyl) (PFO) (PFO-POSS) were synthesized. Compared with the corresponding parent polymers, MEH-PPV and PFO, MEH-PPV-POSS and PFO-POSS have better thermal stability. MEH-PPV-POSS and MEH-PPV have identical absorption and photoluminescent (PL) spectra, both in solution and as thin films. They also have identical electroluminescent (EL) spectra. Devices made from MEH-PPV-POSS exhibit higher brightness (1320 cd m(-2) at 3.5 V) and higher external quantum efficiency (eta(ext) = 2.2 % photons per electron) compared to MEH-PPV (230 cd m(-2) at 3.5 V and eta(ext) = 1.5 % ph el(-1)). Compared with PFO in the same device configuration, PFO-POSS has improved blue EL emission and higher eta(ext).