Effective two-group gamma-ray spectra have been determined for thermal-neutron capture in sodium, nickel, type-304 stainless steel, and tantalum, as well as for 235 U prompt-fission gamma rays. A seven-group compilation of capture gamma rays was used as the basis for this study. Absorbed dose (uncollided and builtup) in several materials was calculated for varying thicknesses of several intervening shielding materials. The resulting function for each combination was reduced to two exponential functions over a range of 0 up to 560 g/cm2. Effective spectra were determined to be as follows: sodium, 6.09 MeV/capture at 5.5 MeV and 5.74 MeV/capture at 2.0 MeV; nickel, 8.33 MeV/capture at 8.0 MeV and 1.62 MeV/capture at 2.0 MeV; type-304 SS, 5.86 MeV/capture at 8.0 MeV and 1.95 MeV/capture at 2.0 MeV; tantalum, 3.76 MeV/capture at 4.0 MeV and 2.88 MeV/capture at 1.5 MeV; prompt fission, 2.31 MeV/fission at 4.0 MeV and4.92 MeV/fission at 1.25 MeV. These effective spectra reproduce, to within an average absolute deviation of less than 7.4%, the absorbed doses (uncollided and builtup) calculated by the detailed spectra, within the ranges of areal density considered.