The tribological response of nanocrystalline diamond (NCD) coatings to variations in moisture content of the environment was examined under reciprocating sliding conditions. Surface roughness, structure, composition, and carbon chemistry were determined by atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy. All coatings were nanocrystalline with crystallite sizes ranging from to 4 to 60 nm as measured by XRD. Visible wavelength Raman spectroscopy of the NCD coatings revealed various carbon chemistries. The NCD coatings could be classified in three subsets by Raman microscopy: (1) coatings exhibiting a single strong peak at 1332 cm^-1 typical of crystalline diamond bonding; (2) coatings possessing broad peaks at 1340 cm^-1 and 1550 cm^-1 characteristic of the D and G peaks in sp² hybridized carbon along with the diamond 1332 cm^-1 peak; and (3) coatings displaying additional peaks at 1135 cm^-1 and 1470 cm^-1, commonly attributed to trans-polyacetylene bonding, along with the three previously described peaks. Reciprocating sliding tests using sapphire counterfaces in controlled humid environments resulted in low friction values for all coatings, between 0.02 and 0.09. Decreasing the humidity caused an increase in the number of cycles to run-in to low friction. These observed differences in run-in and counterface wear as a function of environment will be presented and related to NCD coating composition and microstructure.