A systematic study of the effect of single wall carbon nanotubes (SWCNTs) on the enhanced piezoresistive sensitivity of polyimide nanocomposites from below to above percolation was accomplished. The maximum piezoresistive stress coefficient (Pi) of 1.52 x 10(-3) MPa-1 was noted at just above the percolation threshold concentration (Phi similar to 0.05 wt %) of SWCNT. This coefficient value exceeds those of metallic piezoresistive materials by two orders of magnitude (4.25 x 10(-5) MPa-1 for aluminum). The high piezoresistive characteristics appear to originate from a change in the intrinsic resistivity of the composite caused by the variation of the tunneling distance between conducting inclusions (SWCNTs) under compression or tension. (C) 2009 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys 47: 994-1003, 2009