An approximate analytical solution to the non-equilibrium radiation transport equation is obtained for a gaseous slab consisting of ions and electrons. The analytic expression for the spectral flux escaping the slab exhibits line inversion. The analytic results provide a convenient alternative to numerical integration of the transport equation and in addition provide insight about the physical roles of spatial diffusion and frequency diffusion effects in the transport of radiation through optically thick plasmas. The analytic formalism is applied to a semi-infinite slab and the results show the evolution of an inversion line into an absorption line. The problem of radiation incident on a slab is solved and the results show a peaking of the back scattered spectral flux. Conditions are derived for the onset of induced transparency of a slab to incident radiation.