The optical properties of copper-based alloys containing Zn, Al, Ga, or Ge were investigated using compositional modulation. This method enhances the structure in the spectral reflectivity of materials by measuring the difference in the reflectivities of two alloys. The energy of interband transitions between the d bands and the Fermi level remains approximately constant when up to 2-at.% Zn, Al, or Ga is alloyed to copper but increases for higher solute concentrations. For Cu-Zn, Cu-Al, and Cu-Ga alloys it was found that for a given solute concentration the transitional energy between d bands and the Fermi level is identical. The results are interpreted by assuming that the d bands, as well as the Fermi level, are raised by alloying. Cu-Ge alloys differ in many respects from the behavior of the aforementioned alloys. © 1973 The American Physical Society.