The effects of H2O vapor introduced during focused ion beam (FIB) milling of diamond(100) are examined. In particular, we determine the yield, surface morphology, and microstructural damage that results from FIB sputtering and H2O-assisted FIB milling processes. Experiments involving 20 keV Ga+ bombardment to doses ∼1018 ions/cm2 are conducted at a number of fixed ion incidence angles, θ. For each θ selected, H2O-assisted ion milling shows an increased material removal rate compared with FIB sputtering (no gas assist). The amount by which the yield is enhanced depends on the angle of incidence with the largest difference occurring at θ=75°. Experiments that vary pixel dwell time from 3 μs to 20 ms while maintaining a fixed H2O gas pressure demonstrate the additional effect of beam scan rate on yield for gas-assisted processes. Different surface morphologies develop during ion bombardment depending on the angle of ion incidence and the presence/absence of H2O. In general, a single mode of ripples having a wave vector aligned with the projection of the ion beam vector forms for θ as high as 70°. H2O affects this morphology by lowering the ripple onset angle and decreasing the ripple wavelength. At high angles of incidence (θ≫70°) a step/terrace morphology is observed. H2O-assisted milling at θ≫7- - 0° results in a smoother stepped surface compared with FIB sputtering. Transmission electron microscopy shows that the amorphized thickness is reduced by 20% when using H2O-assisted FIB milling. © 2003 American Vacuum Society.