A procedure for calculating radiation transfer along strongly nonisothermal paths is presented successively. It is orientated to the estimation of transmission and self-radiation of the products of combustion of hydrocarbon fuels. The procedure is based on a new approach to averaging spectral parameters by introducing distributions lines in a given spectral interval over the energies of lower states and matrix elements. Practically it means that the combination of all spectral lines in the preset frequency is substituted by a combination of J groups of lines with averaged values of quantum mechanical characteristics. This approach is intermediate between one-group description with the use of s/d, gamma/d and exact account of each line. A multigroup model is generalized for the cases of vibrational nonequilibrium state and calculations of transmission within the limits of a filter. An error of calculation of the spectral characteristics of molecular gases in the spectral interval of 2-25 mu m does not exceed 10% for a two-group model, i.e. by means of six parameters, in the temperature range T = 250-1000 K, and 12% for a three-group model, i.e. by means of 9 parameters, in the temperature range T = 250-2500 K. The calculated parameters of the three-group model are presented for CO2 in the 4.3 and 2.7 mu m bands, CO in the 4.6 mu m band and H2O in the range 2-10 mu m. (C) 2000 Elsevier Science Ltd. All rights reserved.