  Radiative transfer effects due to overlapping X-ray lines in a
high-temperature, optically-thick, highly-ionized medium are investigated. One
particular example, where the O VIII Ly-alpha doublet coincide in frequency
with the N VII Ly-zeta lines is studied in detail to illustrate the effects on
the properties of the emergent line spectrum. We solve the radiative transfer
equation to study the energy transport of resonance line radiation in a static,
infinite, plane-parallel geometry, which is used to compute the
destruction/escape probabilities for each of the lines for various total
optical thicknesses of the medium, as well as destruction probabilities by
sources of underlying photoelectric opacity. It is found that a large fraction
of the O VIII Ly-alpha line radiation can be destroyed by N VII, which can
result in an reversal of the O VIII Ly-alpha/N VII Ly-alpha line intensity
ratio similar to what may be seen under non-solar abundances. Photoelectric
absorption by ionized carbon and nitrogen can also subsequently increase the
emission line intensities of these ions. We show that line ratios, which are
directly proportional to the abundance ratios in optically thin plasmas, are
not good indicators of the true CNO abundances. Conversely, global spectral
modeling that assumes optically thin conditions may yield incorrect abundance
estimates when compared to observations, especially if the optical depth is
large. Other potentially important overlapping lines and continua in the X-ray
band are also identified and their possible relevance to recent high resolution
spectroscopic observations with Chandra and XMM-Newton are briefly discussed.
