Neutral species (molecules, atoms and radicals) present in rich C2H2/O2premixed flat flame stabilized under reduced pressure (20 torrs) have been analyzed by the coupled molecular beam-mass spectrometry technique. To investigate whether particular variations are observed at the sooting limit (φ = 2.1) six flames of equivalence ratio respectively equal to 1.8, 2.0, 2.1, 2.2, 2.6 and 3.0 have been studied. Results have been mainly used to compare the role played by polyacetylenic hydrocarbons and polycyclic aromatic hydrocarbons (PAH) in the mechanism of soot formation. Main features of the profiles of mole fraction and mass fraction flux versus distance to the burner show that polyacetylenics cannot be considered as active intermediates. On the other hand a strong reactivity is observed at the boundary between oxidation and soot formation zones for PAH. If PAH play an active role in soot formation, it is worth knowing the nature of the species leading to the production of the first aromatic rings. For (his purpose variation of the profiles of mass fraction flux for species containing one to six carbon atoms has been examined as well as the role played by deshydrogenation reactions. Results suggest that C4H4may play a major role in the first step of the mechanism of soot formation.