A new method of accounting for spectral line mixing in calculations of absorption band profiles in rotation-vibration spectra of molecular gases is used to find atmospheric transmission functions at tangential paths in the O-3 (9.6 mu m) and CH4 (3.3 and 7.6 mu m) absorption bands. It is shown that the line mixing in the nu(1), nu(3) O-3 bands and nu 4(,) nu(3) CH4 bands can cause the transmission functions to change by 1-3% and 15%, respectively, as the sighting height ranges from 5 to 30 km.A comparison between calculation results obtained for the 3.3-mu m CH4 band and data measured with the ATMOS interferometer demonstrates their good agreement.