Recently calculated electron impact excitation rates for transitions in helium-like Cl XVI are used to derive the electron temperature sensitive emission line ratio G = ((x + y + z)/w) and the electron density sensitive ratio R = (z/(x + y)) where w, x, y and z are the resonance ls2 1S0 - ls2p 1P1, intercombination ls2 1S0 - ls2p 3P2,1 and forbidden ls2 1S0 - ls2s 3S1 transitions, respectively. These calculations, which differ from those of previous authors, are compared with experimental R and G ratios obtained from X-ray spectra from the JET (Joint European Torus) plasma, for which the electron temperature and density have been measured by independent methods. The R ratio is approximately in its low density limit (R0) for this device and is therefore not electron density sensitive. However it does function as an electron temperature diagnostic as does the G ratio. A comparison is also made with the previously reported results from the Alcator-C tokamak, where the experimental results lie in the electron density sensitive region. The results show good agreement between theory and observation, with discrepancies of typically 7% in R and 10% in G for JET, implying that the theoretical results may be applied to the analysis of remote plasma sources in order to derive values for Te and Ne where no independent estimates for these parameters exist. A departure from coronal equilibrium due to diffusion effects is taken into account for the JET plasma conditions, and the resulting non-coronal ionisation balance is found to produce changes to the theoretical ratios in regions removed from the centre of the JET plasma.