Prompt neutron decay constants have been measured for a series of polyethylene moderated subcritical assemblies. Values of keff varying between 0.20 and 1.0 were obtained by changing the physical size rather than by changing the poison concentration. The decay constants, as determined by the 1/v poison removal method, in a four-group diffusion calculation employing a group dependent buckling, agree to within 10% of the measured values. Preliminary integral type measurements of the neutron spectrum which exists in the assembly during the persistent spatial mode decay indicate that the spectrum is extremely “diffusion cooled.” A simple two-group calculation shows that the decay constant in a subcritical system is proportional to the difference of two spectra. The first is the spectrum which would exist in the assembly when excited by a time independent high energy source; the second is the spectrum existing in the assembly during the persistent mode decay of the neutron density. The conventional description of far-subcritical systems in terms of reactivity is tenuous because of the lack of well defined experiments for its determination. It is apparently more useful to characterize a far-subcritical system by its decay constant, which is directly observable.