Alkylamine-capped gold nanocrystals have been used as chemically sensitive resistors for the detection of volatile organic mercaptan vapors. Thin (< 1 x 10(-4) cm) films of dodecylamine-capped, 6-7 nm diameter Au nanocrystals produced dc electrical resistances of 10 kOmega to 10 MOmega when deposited onto interdigitated Au/Cr electrodes. These chemiresistive vapor detectors displayed a reversible increase in dc electrical resistance when exposed to non-thiol-containing vapors such as water, acetone, or toluene delivered at a constant fraction (0.05) of their vapor pressure, with relative differential resistance responses to these vapors of approximate to5% under such conditions. In contrast, the amine-capped An nanocrystal films exhibited a much larger, irreversible decrease in resistance upon exposure to vapors possessing the thiol (-SH) functionality, including H2S, CH3SH, and propanethiol. Optical spectroscopic and transmission electron microscopy data indicated that the thiols displaced the amine caps, allowing the gold cores to move closer together and in some cases enter into contact, thereby lowering the film resistance. Consistently, the resistance of such films did not decrease upon exposure to octanethiol. For CH3SH, the rate of resistance change under repeated experimental conditions allowed extraction of the concentration of analyte over the range 4 ppb to 1.5 ppm in air.