Spectrometry measurements of neutrons from triton burn-up can provide information on fast ion confinement in deuterium plasmas. This study concerns the relationship between the burn-up neutron spectrum and the triton slowing-down distribution under classical conditions that are affected by certain additional loss mechanisms. Calculations are presented that illustrate the sensitivity of the neutron burn-up spectrum to changes in the confinement properties of the plasma. The level of statistical accuracy of neutron spectral measurements that could be attained in present and planned next-step tokamak experiments is assessed as well as the feasibility for this type of triton burn-up diagnostic.