Empirical data concerning fission product gamma spectra after reactor shutdown were examined. Data were ample for times long after shutdown and for long irradiation times, when long-lived nuclides predominate. However, the earlier times, which are critical to the function of the emergency core cooling system (ECCS), are ultimately more important. A simplified method of estimating gamma spectra was used, which involved sorting nuclides with known gamma spectra into “boxes” on the basis of simple nuclear systematics. A normalized spectrum for each nuclide was created from the relative intensity of each gamma energy. Nuclides were sorted by oddness or evenness of the neutron or proton number, distance from magic numbers, and distance from beta stability. In all sortings, the standard deviations of energy groups were quite large, primarily due to the fact that gamma spectra of most nuclides have a few strong lines rather than a series of many weak lines. Composite spectra were formulated from the individually sorted spectra by weighting the average relative intensities. An optimal combination of weights was derived from the composite spectra; this combination of weights was relatively independent of the number of energy groups used and the size of the magic number “bandwidth.” The optimal width for odd-evenness was usually about twice that for distance from magic number, while the weight for distance from beta stability was negligible. Final spectra representative of a specific reactor model were obtained by applying composite spectra to those fission products that contribute significant gamma activity after shutdown. For a fixed time after shutdown, final spectra for long irradiation times were more smooth than for short irradiation times. The most notable feature was a decisive shift toward softer, but less smooth, spectra as time after shutdown increased. This shift is fortunate for decay heat removal purposes, since relatively harder spectra are present before the ECCS comes into service.