Recent calculations of electron and proton impact excitation rates in Ne VI are used to derive the intensity ratios of lines in the ~402–1006 Å wavelength range as a function of electron temperature (Te) and density (Ne). These results are presented in the form of ratio-ratio diagrams, which should in principle allow both Ne and Te to be deduced for the Ne VI line emitting region of a plasma. Electron temperatures and densities derived from ratio-ratio diagrams involving the 562.7, 997.4, 999.6 and 1006.1 Å lines, in conjunction with observational data for a sunspot obtained with the Harvard S-0555 spectrometer on board Skylab, are found to be compatible, and in good agreement with plasma parameters determined using other methods. This provides some support for the diagnostic calculations presented in this paper, and hence the atomic data used in their derivation. However agreement between theory and observation is very poor for other Ne VI lines in the sunspot spectrum, and for most transitions observed in S-0555 active region and flare data, which is probably due to blending with lines from N III, Mg VI and Mg VII. The application of the calculations to non-solar EUV sources is discussed.