This paper develops a model for calculating radiant heat transfer from smoke layers in fires within enclosures. Nongray, nonhomogeneous, and nonisothermal effects are quantified in a well defined model. Input parameters for the model are the vertical distributions of temperature, product species and enclosure dimensions. A convenient new instrument for accurate measurements of soot concentrations in hostile environments is described. Local mean absorption coefficients for the smoke are calculated by accounting for molecular band effects and an assumed, 1/λ dependence for the spectral absorption coefficient for soot. Calculations are in good agreement with measurements of mean absorption coefficient. Vertical distributions of soot and gaseous species concentrations are found by relating the degree of air entrainment to the measured temperature profile. Simple numerical and analytic approximations are provided which permit an evaluation of the radiative flux by means of an equivalent homogeneous smoke layer model. These approximations allow considerably simplified calculations with little loss in accuracy. The radiance and radiative flux calculations are in good agreement with experimental measurements in a variety of enclosure fires.