Molecular gas radiation calculations have been performed for CO 2 and H 2 O with a line-by-line high-resolution transmission molecular absorption database, with and without particle scattering. Both line-by-line (Monte Carlo) and K-distribution calculations were conducted. The results confirmed that, if the absorption-coefficient distribution function is sufficiently resolved, essentially line-by-line accuracy can be achieved. In addition, comparison was made between line-by-line results and an exponential wideband model combined with Elsasser narrowband model predictions to assess real gas property effects on radiative heat transfer