The antiferromagnetic energy gap and optical properties of single crystals of Cr and of Cr alloyed with 0.45, 0.94, 2.5, and 4.5% Mn and with 1.05 and 2.0% Re have been investigated by measuring the optical absorption over a photon energy range of 0.08-5 eV. The data were taken at 4.2 K by a calorimetric technique. It is found that at 4.2 K, approximately 1% Mn or Re will cause commensurate antiferromagnetic ordering, and the energy gap shifts abruptly from 0.13 to 0.36 eV near this concentration of diluent. Evidence is given for indirect transitions occurring across a double energy gap in the noncommensurate samples, and the results are explained in terms of a band model. The widths of the absorption peak and the peak in ε2 in the commensurate samples are discussed in terms of a gap anisotropy and impurity broadening. Optical absorption peaks near 1, 2, and 3.4 eV are associated with direct interband transitions originating from the N1′, Γ25′, and P4 symmetry points, with a contribution to the 1-eV absorption from transitions from the Fermi surface to the flat unoccupied band extending through N4, Γ12, P3, and H25′.