Composite films of poly(3-hexylthiophene) and single- as well as multi-walled carbon nanotubes are demonstrated to offer a competitive thermoelectric performance. The power factor significantly exceeds values obtained with either constituent alone provided that the conjugated polymer is sufficiently p-doped. The use of single-walled carbon nanotubes consistently results in a higher electrical conductivity with a maximum value above 10 3 S cm-1 and thus gives rise to a power factor of 25 ± 6 μW m-1 K-2 for a filler content of only 8 wt% and a maximum 95 ± 12 μW m-1 K-2 for 42-81 wt%. Moreover, a carbon nanotube content of 8-10 wt% does not compromise the low bulk thermal conductivity of the polymer matrix, which promises a high figure of merit of at least ZT > 10-2 at room-temperature. All samples are cast on plastic substrates, emphasising their suitability for large-area, flexible thermoelectric applications. © 2013 The Royal Society of Chemistry. This work was supported by the Ministerio de Economía y Competitividad through Grant CSD2010-00044 (Consolider NANOTHERM). C.M. gratefully acknowledges financial support from the CSIC through the JAE-Doc program (European Social Fund). M.C.-Q. thanks the Ministerio de Economía y Competitividad for funding through a Ramón y Cajal fellowship. M.S.M.-G. acknowledges the ERC for funding through an ERC 2008 Starting Grant “Nano-TEC” number 240497. Peer Reviewed