The optical band gaps of Ag 3 Au(Se,Te) 2 were determined using diffuse reflectance measurements collected at room temperature. The percentage reflectance was measured with respect to the Spectralon reflectance as 100% reflecting. Figure presents the reflectance as a function of the incident light wavelength in the mid to near-infrared range. All samples showed a strong absorption edge in this spectral range. There was minimal absorption in the visible region, indicating no electronic transitions in that range. For example, x = 0.1 sample showed reflectance above 0.05 between 1250 and 2500 nm, but it dropped to 0.02 near the fundamental absorption onset at 1250 nm due to electron conduction.