We compared the thermomechanical properties, morphologies, gas permeabilities, and electrical conductivities of ultrahigh-molecular-weight polyethylene (UHMWPE) nanocomposite films containing two types of functionalized multiwalled carbon nanotubes (functionalized MWNTs). Both 2-hydroxyethyl triphenyl phosphonium-MWNT (Ph3P-MWNT) and 1,1,1,3,3,3-hexafluoro-2-phenyl-2-propanol-MWNT (CF3-MWNT) were used as reinforcing fillers in the fabrication of UHMWPE hybrid films. UHMWPE nanocomposites with various functionalized MWNT contents were solution-cast to produce the films. The thermomechanical properties and morphologies of the UHMWPE hybrid films were then characterized using differential scanning calorimetry, thermogravimetric analysis, electron microscopy, and mechanical tensile analysis. Transmission electron microscopy studies showed that some of the MWNT particles were dispersed homogeneously within the polymer matrix (on the nanoscale), whereas others were agglomerated. We also found that the addition of only a small amount of functionalized MWNTs was sufficient to improve the thermomechanical properties and the gas barrier of the UHMWPE hybrid films. Even, those hybrid films with low functionalized MWNT contents (i.e., <1 wt%) were found to exhibit much better thermomechanical properties than the pure UHMWPE films. On the other hand, the values of the electrical conductivity remained constant, regardless of the amount of functionalized MWNTs. POLYM. ENG. SCL, 49:2168-2178, 2009. (C) 2009 Society of Plastics Engineers