The fabrication of an extremely‐low‐loss glass optical waveguide, having as little as 4 dB/km total attenuation, has allowed interpretation of the absorption spectrum to a much greater degree than previously possible. It is shown that, beyond about 700 nm, intrinsic ultraviolet absorption will have no effect. Between 700 and 1100 nm, all absorption can be accounted for on the basis of OH to within ±0.7 dB/km. Marked reduction of the water content will leave the scattering as the major loss mechanism, thereby permitting total attenuations of about 2 dB/km.