JET has been operating with deuterium plasmas for the past two years, during which time neutron spectrometry has been applied as a diagnostic technique for determining the ion temperature for suitable discharges. Only ohmically heated discharges and those with ICRF additional heating provide the necessary thermal equilibrium in the deuterium ions for a simple interpretation of the neutron energy spectra to be possible. Furthermore, only plasma discharges generating in excess of 1012 neutrons per second provide a sufficiently strong signal for the statistical accuracy of the measurement to be useful. Combining the "neutron temperature" with the neutron yield determination made with a fission counter diagnostic permits the central deuteron to electron density ratio (nd/ne) to be determined. It is found that this ratio, for Ohmically heated discharges, appears to fall with ion temperature from about 70% at 2keV to 40% at 3keV. This result can be expressed in a more familiar way by stating that Zeff increases from 2.5 to 4.5 over this temperature range. The addition of ICRF heating does not alter the density ratio. These findings are consistent with the observation that high temperatures achieved with Ohmic heating in Tokamaks are correlated with high impurity levels.