Complex refractive indices of a series of SiO2‐ and GeO2‐based glasses were measured to evaluate optical losses of hollow waveguides made of these glasses. Measured optical constants follow the well known dispersion formula and vary with the percentage of Si or Ge atoms to the whole cations. By using empirically defined relations between oscillation peaks and compositions, we can predict refractive indices and waveguide losses theoretically for the unknown SiO2‐ or GeO2‐based glasses.