Observations ofH-like Ne X and He-like Ne IX soft X-ray emission line spectra have been made on the JET (Joint European Torus) tokamak. Recently calculated electron impact excitation rates for He-like neon are used in the derivation of the electron temperature sensitive line ratios G = [(x + y + z )/w], R1 = (1s3p 1P1 Æ1s2 1S0)/w, Lya/w, and the electron density sensitive ratio R = [z/(x + y)], where w, x, y and z are the resonance line (1s2p 1P1 Æ 1s2 1S0), intercombination (1s2p 3P2 ,1 Æ 1s2 1S0) and forbidden lines (1s2s 3S1Æ 1s2 1S0) respectively and Lya. is the H-like Lymana transition. Extensive modelling is carried out to allow for non-coronal conditions due to diffusion effects in these calculations, which are compared with experimental ratios from JET, where independent temperature and density measurements are available. Departures from equilibrium are found for both ohmic and additionally heated plasmas, resulting in significant changes to the G and Lya/w ratios. The R ratio is approaching its upper density limit and is relatively unaffected by non-coronal conditions. Good agreement between the experimental and calculated ratios is found, giving an indication of the accuracy of the combination of atomic data and transport employed in the calculations. This implies that the the theoretical results may be applied to the analysis of remote plasma sources for derivation of these parameters.