Reciprocating sliding tests of sapphire balls against the nanocrystalline diamond have been performed to examine the effects of moisture in the surrounding ambient environment. Microstructure of the diamond coatings was confirmed by X-ray diffraction (XRD) to be nanocrystalline with crystallite sizes ranging from to 20 to 70 nm, with a preferred texture in the (220) orientation. These diamond coatings had similar visible wavelength Raman absorption bands that were deconvolved into 5 peaks: a single sharp peak at 1332 cm^-1 typical of crystalline diamond; broad peaks at 1340 cm^-1 and 1580 cm^-1 characteristic of the D and G peaks in sp² hybridized carbon; and, additional peaks at 1135 cm^-1 and 1470 cm^-1 that are commonly attributed to transpolyacetylene bonding. All nanodiamond coatings initially possessed high friction (0.5 - 0.8) before running-in to a steady-state value of ~0.05; each coating had a characteristic run-in profile to low friction (<0.1) occurring between 300 to 1000 cycles. Coatings with larger crystallites and higher surface roughness were more sensitive to changes in humidity, with faster run-in occurring as humidity increased between 3 and 45%. For these larger-crystallite coatings, wear of the sapphire counterface increased by an order of magnitude between 5 and 45% RH. However, coatings having smaller grain sizes and lower roughness exhibited similar run-in and counterface wear regardless of humidity. Coating tribological performance as a function of humidity will be related to the microstructural and compositional differences among the nanocrystalline diamond coatings.