Zinc oxide (ZnO) nanoparticles decorated single walled carbon nanotubes (SWNTs) were electrochemically synthesized where the deposition conditions were systematically explored to tailor the size, density, and microstructure of the ZnO nanoparticles and correlated to the gas sensing performance. Room temperature conductometric detection of various analytes including CO, CO2, NO2, NH3, SO2, H2S with ZnO/SWNT hybrid nanostructures demonstrated uncharacteristic selectivity towards H2S with little to no response for the other analytes examined. Optimal ZnO/SWNTs gas sensor devices showed a significantly increased in H2S sensitivity over unfunctionalized SWNT networks (i.e. 4.96 % per ppmV vs. 0.225 % ppmV) with a lower detection limit in the ppb range. Additionally, the H2S sensing performance was greatly improved by enhancing the crystallinity of ZnO nanoparticles.