The effect of replacement of 15% of N-2 by CO2 (on a mole basis) in unburnt mixtures on the structure of laminar premixed methane/oxygen/nitrogen and propane/oxygen/nitrogen flames stabilized on a flat burner at 1 atm is studied. The emphasis of this work is on the effect of CO2 addition on the mole fractions of intermediates that are potential precursors of polycyclic aromatic hydrocarbons (PAH). Mole fraction profiles of reactants (CH4, C3H8, O-2, and CO2), major intermediates (CH3, CH4, CH2O, C2H2, C2H3, C2H4, C2H6, C3H3, and C3H4) and final products (H2O, CO, CO2, and H-2) in fuel-rich (phi = 1.2) CH4/O-2/N-2, CH4/O-2/N-2/CO2, C3H8/O-2/N-2, and C3H8/O-2/N-2/CO2 flames were measured using flame-sampling molecular beam mass spectrometry and calculated using the PREMIX code and three detailed chemical kinetic mechanisms reported in the literature for combustion of small hydrocarbons: AramcoMech 1.3, Konnov's mechanism, and GRI-Mech 3.0. Temperature measurements in the flames were carried out using the thermocouple technique. The measurements showed that the replacement of 15% of N-2 by CO2 in unburnt mixtures containing either methane or propane leads to a reduction in the postflame temperature and a decrease in the maximum mole fraction of the majority of intermediate species in the reaction zone. The performances and deficiencies of the mechanisms in predicting the new experimental data are discussed. In addition, the calculations carried out allowed us to distinguish both the chemical and thermophysical influences of CO2 on the chemical structure of the flames studied. The chemical effect of CO2 was found to be stronger in the flame of methane than in the propane flame. This was explained by the different reactivity of the fuels considered. Analysis of the AramcoMech 1.3 and Konnov's reaction mechanisms was carried out to gain insight into how the addition of CO2 to the unburnt mixtures effects the kinetics of methane and propane combustion. The analysis showed that CO2 addition changed the rates of reactions involving H and OH radicals, mainly the CO + OH (sic) CO2 + H reaction, resulting in an increase in the OH/H ratio in the reaction zone of the flames. This, in its turn, leads to enhanced formation of small oxygenates and reduced production of hydrocarbons that are typical PAH precursors.