Two natural diamond detectors (NDDs) operating at room temperature were used for Fusion Neutronics Source (FNS) deuterium–tritium (DT) neutron spectra measurements at different points around the tritium target and for different deuteron beam energies. Energy resolution of both NDDs were measured, with values 1.95&percent; and 2.8&percent;. Due to the higher energy resolution of one of the two NDDs studied it was possible to measure the shape of the DT neutron energy distribution and its broadening due to deuteron scattering inside the target. The influence of pulse pileup on the energy resolution of the combined system (NDD+electronics) at count rates up to 3.8×105counts/s was investigated. A 3.58&percent; energy resolution for the spectrometric system based on NDD and a 0.25 &mgr;s shaping time amplifier has been measured at a count rate of 5.7×105counts/s. It is shown that special development of a fast pulse signal processor is necessary for NDD based spectrometry at count rates of approximately 106counts/s. ©1997 American Institute of Physics.