Significant variations in the attenuation characteristics of hollow alumina waveguides (suitable for use in the manufacture of CO2waveguide lasers) have been measured over the9-11 mum waveband. Existing waveguide transmission theory has been successfully used to explain the qualitative nature of the observed experimental results by taking into account the effects of dispersion due to active lattice absorption bands in alumina. The suprising conclusion of the work is that the major variations in attenuation are a result of a complete change in the guiding nature of the waveguide atsim 10 mum. At this point, the nature of the waveguide changes from being "leaky" at shorter wavelengths to exhibiting attenuated total internal reflection at longer wavelengths. This results in there being a distinct difference in attenuation characteristics for the two main laser emission bands centered at 9.6 and 10.6 μm, respectively.