A new class of conductive composites with good gas sensitivity was fabricated by filling polystyrene with vapor grown carbon nanofibers (VGCNF). A solution mixing/solvent removal procedure was used. VGCNFs form conductive networks at fiber loadings above the percolation limit within the matrix. Greatly improved conductivity is achieved relative to the same volume fraction of carbon black addition when these fibers are distributed to give reasonably uniform dispersions in the matrix. The high aspect ratios of these fibers (similar to 70-250 mn diameters and 5-75 mu m lengths) assist in forming low wt.% percolation thresholds (below 1 wt.% fiber). Excellent gas sensitivity with 10(4)-10(5) times higher than the original resistance value in many saturated organic vapors and a maximum resistance response of about 1.1 x 10(5) times exposure to saturated THF vapor at 6.25 wt.% of VGCNF in the polystyrene matrix was observed. The maximum resistance response declined from about 2.0 x 10(5) times at 15 degrees C to about 3.4 x 10(4) times at 55 degrees C. These composites exhibited stable and reusable gas sensitivity to THF vapor. Carbon black/polystyrene composites exhibit a negative vapor coefficient (NVC) upon swelling caused by filler redistribution. In contrast, VGCNF/polystyrene composites are more stable, with much smaller NVC values due to their high aspect ratios and reinforcing effects which stabilize electrical percolation pathways. Thus, VGCNF/organic polymer composites are good gas sensor candidates for detecting organic vapors. (c) 2005 Elsevier Ltd. All rights reserved.