The effects of molecular weight on electroluminescent properties of the light emitting polymeric nanocomposite, poly(9,9-dioctylfluorene-alt-thiophene) with chemically bonded gold nanoparticles (PDOFT-Au), have been studied under a condition of constant polymer-to-gold weight ratio. The polymer, PDOFT, was first synthesized via the Suzuki cross-coupling reaction and then bonded to in situ-formed gold nanoparticles (AuNPs) via terminal thiol functional groups which had been generated during the quenching of polymerization. A series of PDOFT-Au's of various molecular weights have thus been synthesized. At a constant polymer-to-gold weight ratio, the average size of gold nanoparticles (AuNPs) increased with an increase in the molecular weight of PDOFT. Although an increased molecular weight led to a red shift in UV-absorption and PL spectra as well as an increased PL quantum efficiencies (Phi(PL)) for all samples (both PDOFT and PDOFT-Au), the effect of AuNPs bonding became more noticeable when the molecular weight was higher. As for light emitting diode (LED) device fabrication, an increase in the molecular weight of PDOFT also led to a red shift in the EL spectra of the fabricated LED devices. Nevertheless, PDOFT-Au, compared with PDOFT, had a lower threshold voltage, an increased brightness and current density, and an improved photometric efficiency. Moreover, the photometric efficiency increased with an increase in the molecular weight of the polymer, from 0.298 cd/A for PDOFT-Au3 up to 0.645 cd/A for PDOFT-Au1. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3972-3979, 2009