  It is shown that, in addition to the Thomson scattering, the absorption due
to the electron-electron, electron-ion and the electron -atom collisions in a
partially ionized cosmic plasma would also contribute to the optical depth of
the cosmic microwave background (CMB). The absorption depth depends on the
plasma temperature and frequency of the CMB radiation. The absorption effects
are prominent at the low frequency part of the CMB spectrum. These effects when
included in the interpretation of the CMB spectrum may necessitate a revised
view of the ioniziation of the universe.
