[en] The spectral impurities of the EG and G 25-kCi ⁶⁰Co source were determined by using a 1-mCi mockup source. The mockup source was identical in construction to the 25-kCi source and was identically mounted. The spectrum of the mockup source was measured by using a 3 x 3 inch NaI detector and a multichannel analyzer. Measurements were made with and without a graded Z filter. ''Pure'' ⁶⁰Co spectra, i.e., spectra free of scattered impurities, were used to subtract out the direct 1.17- and 1.33-MeV lines from the mockup spectra to leave only impurities. Several checks on the resulting impurity spectra indicated that a conservative estimate on their accuracy is +- 20%. A large fraction of the total number of x rays was found to be due to spectral impurities which caused the average energy of the spectra to be appreciably degraded. The average energy of the filtered spectra is 1.105 +- 0.020 MeV, while that of the unfiltered spectrum is 0.937 +- 0.020 MeV