The influence of real gas radiation on the thermal and hydrodynamic stability of a two-dimensional layer of radiatively participating H2O and/or CO2 heated from below is investigated. The nongray radiation effects of the two species are treated rigorously using a global spectral approach, the Spectral Line Weighted-sum-of-gray-gases model. The phenomena are explored by solving the full coupled laminar equations of motion, energy, and radiative transfer from the low-Rayleigh number, pure conduction-radiation regime through the onset of buoyancy-induced flow to the developed Bénard convection regime. The evolution of the thermal, hydrodynamic, and radiative heating fields is studied, and the critical Rayleigh number is characterized as a function of participating gas species mole fraction, average layer gas temperature, layer thickness, wall emissivity, and total pressure. It is found that participating radiation in the medium has the effect of stabilizing the layer, delaying transition from a stable conduction regime to buoyancy-induced flow. The development of convective flow and temperature, along with the radiative heating are presented. It is found that the critical Rayleigh number in the radiatively participating gas layer can be more than an order of magnitude higher than the classical convection-only scenario. The onset of instability is found to depend on the species mole fractions, average gas temperature in the layer, wall emissivity, layer thickness, and total pressure. Generally, all other variables being the same, H2O has a greater stabilizing influence on the layer than CO2.