Passive emission spectroscopy using a line by line model carbon dioxide around band head region has been developed to determine the temperature and concentration fields of exhaust gases. Resulting is a model which takes into account the hot band parameter in the first CO2 band head in the 2397 cm-1 region. Modeling absorption spectra have been compared with measurement absorption spectra in a new tight high temperature gas cell. Error is less than 2 percent until 1 100 K. The solution of the radiative transfer equation, is found by least square fitting between measured spectra and line by line modeling spectra inversion software, uses a non-linear simplex algorithm and a procedure of random couple pulling of temperature and partial pressure for testing convergence. It allows a discretization in four elements of the gas flow. Measurements were performed with a FTIR spectrometer using a telescope form gas exhaust form a 100 KW engine through an ambient air of 7 meters path. The average value is in good agreement with the measure of a thermocouple, in the center of the flow positioned. Thermocouple maximum deviation is 23 K between the measures and the worst element calculation.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.