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Previous laboratory results have suggested that unheated TiO2 nanowire arrays provide a means to detect trace level concentrations of nitro-group containing compounds in the gas phase with fast response and high chemical selectivity. The detection method is based on the chemiresistive effect, where an electron-deficient compound adsorbed to the surface of an n-type semiconductor causes a surface charge deficit on the semiconductor leading to an increase in its resistance. However, a major issue with this method is that chemiresistive sensors based on TiO2 are also sensitive to the presence of water vapor, and this cross-sensitivity could lead to artifacts for sensors used under environmental conditions. Results are presented here, where thin planar arrays of TiO2 nanowires were tested to determine the sensitivity towards water vapor, along with the detection limits and response times towards several nitro-containing organic molecules as a function of gas-phase water vapor concentration. When water vapor concentrations were carefully controlled to ensure they remain constant throughout the testing cycle, it was found that TiO2 nanowire devices could detect 2,4,6-trinitrotoluene at a concentration as low as a few tens of part per trillion by volume with a response time in the range of $10^{2}$ – $10^{3}$ s, depending on experimental conditions.