The use of H2O vapor as a chemical adjunct for focused ion beam micromachining has been investigated. The presence of H2O vapor during micromachining with a 25 keV Ga+ beam increases the removal rate of carbon-containing materials such as polyimide, PMMA, and other resists by a factor of 20 (relative to physical sputtering), and that of diamond by a factor of 10. In addition, H2O causes a decrease in the removal rate of some other materials (e.g., Si and Al) by as much as a factor of 10, effectively increasing the selectivity of polymers over these other materials by as much as a factor of 200. The dependence of the removal rate on H2O pressure at the sample, pixel dwell time, pixel size, pattern frame time (corresponding to pattern size), and current density has been investigated utilizing PMMA. PMMA removal rates were calculated by measuring the depth of rectangular pits micromachined into PMMA films under the various experimental conditions. In addition to investigating the effect of H2O on material removal rates, the change in removal rate of PMMA in the presence of selected gases including O-2, H-2, and CH3OH, has been measured.