Neutron Emission Spectroscopy (NES) has been used to study JET plasmas produced with auxiliary power (PAUX) during the recent Trace Tritium Experiments (TTE of October 2003), as well as the main Deuterium-Tritium Experiments (DTE1 of 1997). The spectrum of the neutron emission from d + t Æ a + n reactions was measured with the Magnetic Proton Recoil (MPR) neutron spectrometer. The recorded spectra were analysed as a superposition of several contributions related to different velocity components of the fuel ion populations perturbed by PAUX. Information was derived on the kinetic states of the fuel ion populations, such as temperatures and relative fractions of thermal and supra-thermal reactions. In addition, the collective states of the fuel ions, such as toroidal rotation were also measured. Moreover, the MPR count rate was used to determine the total neutron rate, Yn(t), taking into account neutron emission effects as explained in an accompanying paper. While the DTE1 data were subjected to post mortem analysis, the TTE data were analyzed between discharges during the on-going campaign so that direct input could be given to the evolving experimental program on a shot-by-shot basis.