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 O₃ (9.6 μm) and CH₄ (3.3 and 7.6 μm) absorption bands. It is shown that line mixing in the v₁, v₃ O₃ bands and v₄, v₃ 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-μm CH₄ bands and data measured with the ATMOS interferometer demonstrates their good agreement.