The optical properties of &bgr;′ phase CoAl, CoGa, and NiGa were studied as functions of composition and photon energy in the spectral region from 0.7 to 6.3 eV. Similar investigations were conducted on NiAl between 0.7 and 2.0 eV. The specimens were prepared by a mechanical polishing technique that was shown, by the use of annealing studies, to produce surfaces from which reliable optical data could be taken. Measurements were made by the use of two polarimetric techniques on a reflectometer that was designed by the authors. The results of the optical measurements indicate that some of the investigated alloys show free electron behavior in the infrared, although the limited spectral region of investigation did not permit the authors to apply the Drude free electron theory to the data with confidence. The possible presence of hybridized plasma oscillations is indicated by structure in the loss function. The prominent optical absorption structure which is displayed by these materials in the visible and ultraviolet regions of the spectrum, and which indicates the presence of interband transitions, has been interpreted by the use of a simple rigid band model.