Net emission coefficients of radiation were calculated for isothermal plasmas of air and SF6 as a function of the plasma temperature 5,000-30,000 K and the arc radius (0.01-10 cm) at various plasma pressures. Calculations take into account continuum and line radiations, special attention has also been taken to influence of molecular species in case of the air plasmas. It has been found that the molecular bands of O-2, N-2, N-2 (+), NO and NO+ have very strong effect on the net emission coefficients at low temperatures (below 10,000 K). In case of SF6, effect of PTFE admixture on the net emission coefficients was also studied. It follows from the calculations that the net emission coefficients vary very little with various admixtures of PTFE. Values of net emission coefficients if SF6 plasma calculated for various spectral regions were compared.