R-matrix calculations of electron impact excitation rates in Ni XVIII are used to derive theoretical electron-temperature-sensitive emission line ratios involving 3s – 4p, 3p – 4d, 3p – 4s, and 3d – 4f transitions in the 41–53 Å wavelength range. A comparison of these with solar flare observations from a rocket-borne X-ray spectrograph (XSST) reveals generally excellent agreement between theory and experiment (within the experimental and theoretical uncertainties), which provides support for the atomic data adopted in the analysis. However the 3s2S – 4p2P1/2 line of Ni XVIII at 41.22 Å appears to be blended with the Fe XIX 13.74 Å feature observed by XSST in third order. In addition, the measured Ni XVIII intensity ratio I(3p2P3/2 – 4s2S)/I(3p2P1/2 – 4s2S) = I(51.02 Å)/I(50.26 Å) = 0.56, a factor of ∼ 3.8 smaller than the theoretical (temperature and density-insensitive) value of 2.1. The reason for this discrepancy is currently unexplained, but is unlikely to be due to blending of the 50.26 Å line, as the intensity of this feature is consistent with that expected from the other Ni XVIII lines in the XSST spectrum. Future observations of the Ni XVIII lines by the Advanced X-ray Astrophysics Facility (AXAF) should allow this problem to be resolved, and may also permit the use of the lines as electron-temperature diagnostics.