The performance of chemiresistor sensors made from thin film assemblies of single-wall carbon nanotubes, multiwall carbon nanotubes, reduced graphene oxide nanosheets (RGON), and gold nanoparticles (AuNP) was assessed with an immersible microelectrode array. Carbon nanotube and RGON chemiresistors were functionalized with octadecyl-1-amine and the AuNP chemiresistors were functionalized with 1-hexanethiol. The analytes examined were aqueous solutions of petroleum hydrocarbons: cyclohexane, naphthalene, benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX analytes). Titrations were performed to determine the detection limits of the different chemiresistors. The AuNP chemiresistor was the most sensitive to all the analytes with limits of detection between 0.2 and 0.6 ppm in water. In contrast, the multiwall carbon nanotube chemiresistor was the least sensitive to the analytes with limits of detection between 20 and 200 ppm. These sensitivities show that these nanomaterials have the potential, with further optimization, of being incorporated into devices that would respond to hydrocarbons in water at concentrations relevant to the regulations of the US Environmental Protection Agency. The stability of the sensors over 26 days was also assessed. Remarkably, there was a negligible change in the electrical resistance of the RGON sensors over this time. The nanotube sensors increased in resistance and the AuNP decreased in resistance over the same period. The drifting resistances did not affect the sensitivity of the nanomaterials, which remained constant with time. © 2013 Her Majesty the Queen in Right of Australia.